Method for manufacturing an array of sensors on a single chip

ABSTRACT

A nanostructure sensing device comprises a semiconductor nanostructure having an outer surface, and at least one of metal or metal-oxide nanoparticle clusters functionalizing the outer surface of the nanostructure and forming a photoconductive nanostructure/nanocluster hybrid sensor enabling light-assisted sensing of a target analyte.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of U.S. patent application Ser. No.15/297,693 (filed Oct. 19, 2016), which is a Continuation-in-part ofU.S. patent application Ser. No. 13/861,962 (filed Apr. 12, 2013), nowU.S. Pat. No. 9,476,862, which is based on U.S. Provisional PatentApplication Ser. No. 61/623,957 (filed Apr. 13, 2012), Application Ser.No. 61/625,511 (filed Apr. 17, 2012), Application Ser. No. 61/730,865(filed Nov. 28, 2012), and Application Ser. No. 61/775,305 (filed Mar.8, 2013), which applications are all incorporated herein by reference intheir entireties and to which priority is claimed.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This work was supported by the National Science Foundation (NSF) underECCS-0901712 grant, by the Defense Threat Reduction Agency (DTRA) underHDTRA11010107, and by the National Institute of Standards and Technology(NIST) under SB134110SE0579 and SB1341 I 1SE0814. The US government hascertain rights in this invention.

FIELD OF THE INVENTION

The present invention relates to a sensing device including asemiconductor nanostructure and at least one of metal or metal-oxidenanoparticles functionalizing the nanostructure and forming a hybridsensor that enables light-assisted sensing of a target analyte.

BACKGROUND OF THE INVENTION

Detection of chemical species in air, such as industrial pollutants,poisonous gases, chemical fumes, and volatile organic compounds (VOCs),is vital for the health and safety of communities around the world (seeWatson J and Ihokura K (1999) Special issue on Gas-Sensing Materials,Mater. Res. Soc. Bull. 24:14). The development of reliable, portable gassensors that can detect harmful gases in real-time with high sensitivityand selectivity is therefore extremely important (Wilson D M et al.(2001) “Chemical Sensors for Portable, Handheld Field Instruments,” IEEESensors Journal 1:256-274; Eranna G et al. (2004) “Oxide Materials forDevelopment of Integrated Gas Sensors—A Comprehensive Review/IntegratedGas Sensors—A Comprehensive Review,” Critical Reviews in Solid State andMaterial Sciences 29:111-188).

Due to their small size, ease of deployment, and low-power operation,solid-state thin film sensors are favored over analytical techniquessuch as optical and mass spectroscopy, and gas chromatography forreal-time environmental monitoring (Wilson D M et al. (2001), supra,IEEE Sensor Journal 1:256-274; Shimizu Y and Egashira M (1999) “Basicaspects and Challenges of Semiconductor Gas Sensors,” Mater. Res. Soc.Bull. 24:18; Sze S M (1994) Semiconductor Sensors 1^(st) ed, Willey; NewYork). Selectivity, which is a sensor's ability to discriminate betweenthe components of a gas mixture and provide detection signal for thecomponent of interest, is an important consideration for the sensor'sreal-life applicability. Conventional metal-oxide based thin filmsensors, despite decades of research and development (Brattain J B W H(1952) “Surface properties of germanium,” Bell. Syst. Tech. Journal32:1; Azad A M et al. (1992) “Solid-State Sensors: A Review,” J.Electrochem. Soc. 139(12):3690-3704), still lack selectivity fordifferent species and typically require high working temperatures(Meixner H and Lampe U (1996) “Metal oxide sensors,” Sens. and ActuatorsB 33:198-202; Nicoletti S et al. (2003) “Use of Different SensingMaterials and Deposition Techniques for Thin-Film Sensors to IncreaseSensitivity and Selectivity,” IEEE Sensors Journal 3:454-459; Demarne Vand Sanjines R (1992) Gas Sensors-Principles, Operation and Developmentsed. G. Sberveglieri, Kluwer Academic, Netherlands). As such, theusability of such conventional sensors is severely limited and poseslong-term reliability problems.

For a chemical sensor, the active surface area is an important factorfor determining its detection limits or sensitivity. It is known thatthe electrical properties of nanowires (NWs) change significantly inresponse to their environments due to their high surface to volume ratio(Cui Y et al. (2001), supra, Science 293:1289-1292; Zhang D et al.(2004) “Detection of NO ₂ down to ppb levels using individual andmultiple In ₂ O ₃ nanowire devices,” Nano. Lett. 4:1919-1924; Kong J etal. (2000) “Nanotube Molecular Wires as Chemical Sensors,” Science287:622-625; Comini E et al. (2002) “Stable and highly sensitive gassensors based on semiconducting oxide nanobelts,” Appl. Phys. Lett.81:1869). NWs are therefore well suited for direct measurement ofchanges in their electrical properties (e.g. conductance/resistance,impedance) when exposed to various analytes. Substantial research hasdemonstrated the enhanced sensitivity, reactivity, and catalyticefficiency of the nanoscale structures (Cui Y et al. (2001), supra,Science 293:1289; Li C et al. (2003) “In ₂ O ₃ nanowires as chemicalsensors,” Appl. Phys. Lett. 8:1613; Wan Q et al. (2004) “Fabrication andethanol sensing characteristics of ZnO nanowire gas sensors,” Appl.Phys. Lett. 84:3654; Wang C et al. (2005) “Detection of H ₂ S down toppb levels at room temperature using sensors based on ZnO nanorods,”Sens. and Actuators B 113:320-323; Wang H T et al. (2005)“Hydrogen-selective sensing at room temperature with ZnO nanorods,”Appl. Phys. Lett. 86:243503; Raible I et al. (2005) “V ₂ O ₅ nanofibers:novel gas sensors with extremely high sensitivity and selectivity toamines,” Sens. and Actuators B 106:730-735; McAlpine M C et al. (2007)“Highly ordered nanowire arrays on plastic substrates for ultrasensitiveflexible chemical sensors,” Nat Mater 6:379-384).

There have been attempts to demonstrate sensors based onnanotube/nanowire decorated with nanoparticles of metal andmetal-oxides. For example, Leghrib et al. reported gas sensors based onmultiwall carbon nanotubes (CNTs) decorated with tin-oxide (SnO₂)nanoclusters for detection of NO and CO (see Leghrib R et al. (2010)“Gas sensors based on multiwall carbon nanotubes decorated with tinoxide nanoclusters,” Sens. and Actuators B: Chemical 145:411-416). Usingmixed SnO₂/TiO₂ included with CNTs, Duy et al. demonstrated ethanolsensing at a temperature of 250° C. (Duy N V et al. (2008) “Mixed SnO ₂/TiO ₂ Included with Carbon Nanotubes for Gas-Sensing Application,” J.Physica E 41:258-263). Balázsi et al. fabricated hybrid composites ofhexagonal WO₃ powder with metal decorated CNTs for sensing NO₂ (BalázsiC et al. (2008) “Novel hexagonal WO ₃ nanopowder with metal decoratedcarbon nanotubes as NO2 gas sensor,” Sensors and Actuators B: Chemical133:151-155). Kuang et al. demonstrated an increase in the sensitivityof SnO₂ nanowire sensors to H₂S, CO, and CH₄ by surfacefunctionalization with ZnO or NiO nanoparticles (Kuang Q et al. (2008)“Enhancing the photon-and gas-sensing properties of a single SnO2nanowire based nanodevice by nanoparticle surface functionalization,” J.Phys. Chem. C 112:11539-11544). ZnO NWs decorated with Pt nanoparticleswere utilized by Zhang et al., showing that the response of Ptnanoparticles decorated ZnO NWs to ethanol is three times higher thanthat of bare ZnO NWs (Zhang Y et al. (2010) “Decoration of ZnO nanowireswith Pt nanoparticles and their improved gas sensing and photocatalyticperformance,” Nanotechnology 21:285501). Chang et al. showed that byadsorption of Au nanoparticles on ZnO NWs, the sensor sensitivity to COgas could be enhanced significantly (Chang S-J et al. (2008) “Highlysensitive ZnO nanowire CO sensors with the adsorption of Aunanoparticles,” Nanotechnology 19:175502). Dobrokhotov et al.constructed a chemical sensor from mats of GaN NWs decorated with Aunanoparticles and tested their sensitivity to N₂ and CH₄ (Dobrokhotov Vet al. (2006) “Principles and mechanisms of gas sensing by GaN nanowiresfunctionalized with gold nanoparticles,” J. Appl. Phys 99:104302). GaNNWs coated with Pd nanoparticles were employed for the detection of H₂in N₂ at 300K by Lim et al. (Lim W et al. (2008) “Room temperaturehydrogen detection using Pd-coated GaN nanowires,” Appl. Phys. Lett.93:072109).

Although such results demonstrate the potentials of thenanowire-nanocluster based hybrid sensors, fundamental challenges anddeficiencies in such prior attempts remain. Most of the results providefor mats of nanowires. Although such mats may increase sensitivity, thecomplex nature of inter-wire conduction makes interpreting the resultsdifficult. Also, room-temperature operation of such previous sensors hasnot been demonstrated, and the selectivity is shown for only a verylimited number of chemicals. Conventional sensor devices require highoperating temperatures (250° C.) and large response times (more than 5minutes). Indeed, such temperature-assisted sensors typically providefor an integrated heater for the device. Further, the reportedsensitivities of such conventional devices were quite low even with longresponse times. Further, such conventional devices typically do notprovide for air as the carrier gas. However, the ability of a sensor todetect chemicals in air is what ultimately determines its usability inreal-life.

Thus, such demonstrations have resulted in poor selectivity of knownchemical sensors, and therefore have not resulted in commercially viablegas sensors. For real-world applications, selectivity between differentclasses of compounds (such as between aromatic compounds and alcohols)is highly desirable. For example, the threat of terrorism and the needfor homeland security call for advanced technologies to detect concealedexplosives safely and efficiently. Detecting traces of explosives ischallenging, however, because of the low vapor pressures of mostexplosives (Moore, D S (2004) “Instrumentation for trace detection ofhigh explosives,” Review of Scientific Instruments 75(8):2499-2512;Yinon J (2002) “Field detection and monitoring of explosives,” TrACTrends in Analytical Chemistry 21(4):292-301; Senesac L. and Thundat T G(2008) “Nanosensors for trace explosive detection,” Materials Today11(3):28-36. Moreover, the difficulty of explosive detection isaggravated by the noisy environment which masks the signal from theexplosive, the potential for high false alarms, and the need todetermine a threat quickly. As such, trained canine teams remain themost reliable means of detecting explosive vapors to date; however, dogsare expensive to train and tire easily.

An ideal chemical sensor would be able to distinguish between theindividual analytes belonging to a particular class of compounds, e.g.detection of the presence of benzene or toluene in the presence of otheraromatic compounds, detection of a particular explosive compound,detection of a particular alcohol, etc. This is extremely challenging asmost semiconductor-based sensors use metal-oxides (such as SnO₂, In₂O₃,ZnO) as the active elements, which are limited due to the non-selectivenature of the surface adsorption sites. The surface/adsorbateinteractions of conventional sensor structures are limited andnon-specific. Thus, conventional sensor devices lack the sameselectivity as their bulk-counterpart devices.

Accordingly, there is a need for a nanostructure sensor device thatsolves one or more of the deficiencies of conventional devices.

SUMMARY OF THE INVENTION

The present invention is directed to highly selective and sensitivesensor devices including semiconductor nanostructures decorated withmetal and/or metal-oxide nanoclusters or particles. The disclosedsensors provide numerous advantages over conventional sensorsincluding: 1) light-induced room-temperature sensing as opposed tothermally induced sensing, providing for reliable operation atlow-power, longer lifetime, and fast response/recovery time; 2)excellent selectivity of sensing of selected compounds (e.g., sensorsable to distinguish toluene from other aromatic compounds); 3) widesensing range (50 ppb-1%); 4) fast response and recovery; and 5)reliable and repeatable operation.

According to implementations of the present invention, hybridchemiresistive architectures utilizing nanoengineered wide-bandgapsemiconductor backbone functionalized with multicomponent photocatalyticnanoclusters of metal-oxides and/or metals are provided. Suchimplementations, e.g. providing for chip scale hybrid sensorarchitecture backbones, are particularly suitable for mass productionemploying industry standard fabrication techniques, such as forcommercial applications. The sensors operate at room-temperature viaphotoenabled sensing, and utilize standard microfabrication techniques.Thus, economical, multianalyte single-chip sensors are achieved.

According to embodiments of the present invention, the disclosedsemiconductor nanostructures exhibit relatively inactive surfaceproperties (i.e., with little or no chemiresistive sensitivity todifferent classes of organic vapors). The nanostructures arefunctionalized with analyte-dependent active metal-oxides and/or metals.Photoconductive metal-oxide-semiconductors may be utilized as afunctionalizing material due to their active surface properties andlight-assisted sensing operation. Unlike most metal-oxide-based sensorsthat operate at high temperatures, the photoconductive hybrid sensordevices of the present invention enable rapid light-assisted sensing attemperatures well below 100° C., and in particular at temperaturesbetween about 10° C. and 100° C., including at room temperature (e.g.,between about 18° C. and about 24° C.). Thus, the disclosed sensorsoperate at temperatures well below that required by conventional oxidesensors (e.g., requiring sensing temperatures higher than 100° C.),thereby providing rapid sensing capabilities at room temperatureassisted by UV light illumination.

According to one embodiment, a multi-analyte sensor comprises asubstrate having an upper surface, a semiconductor nanostructure havingan outer surface and disposed on the upper surface of the substrate,first metal-oxide nanoparticles functionalizing the outer surface of thesemiconductor nanostructure and enabling detection of a target analytein the presence of light, the first metal-oxide nanoparticles have afirst adsorption profile, and second metal nanoparticles functionalizingthe outer surface of the semiconductor structure. The second metalnanoparticles have a second adsorption profile. The target analytepreferentially adsorbs on the first metal-oxide, and an interferinganalyte preferentially adsorbs on the second metal nanoparticles. Thesensor exhibits a change in output upon detection of the target analyte,the output selected from the group consisting of current, voltage andresistance.

The disclosed sensors enable detection of the target analyte withinvarious carrier gases, including air, nitrogen or argon. Thesemiconductor nanostructure may comprise gallium nitride (GaN), indiumnitride (InN), aluminum gallium nitride (ALGaN), zinc oxide (ZnO),Indium arsenide (InAs). The metal-oxide nanoparticles may comprise oneor more nanoparticles selected from the group consisting of titaniumdioxide (TiO₂) nanoparticles, tin oxide (SnO₂) nanoparticles, zinc oxide(ZnO) nanoparticles, nickel oxide (NiO) nanoparticles, copper oxide(Cu_(x)O_(x)) nanoparticles, cobalt oxide (Co_(x)O_(x)) nanoparticles,iron oxide (Fe_(x)O_(x)) nanoparticles, zinc magnesium oxide(Zn_(1-x)Mg_(x)O) nanoparticles, magnesium oxide (MgO) nanoparticles,vanadium oxide (V_(x)O_(x)) nanoparticles, lanthanum oxide (La₂O₃)nanoparticles, zirconium oxide (ZrO₂) nanoparticles, aluminum oxide(Al₂O₃) nanoparticles, strontium oxide (SrO) nanoparticles, lanthanumoxide (La₂O₃) nanoparticles, cerium oxide (Ce_(x)O_(x)) nanoparticles,praseodymium oxide (Pr_(x)O_(x)) nanoparticles, promethium oxide (Pm₂O₃)nanoparticles, samarium oxide (Sm₂O₃) nanoparticles, europium oxide(Eu₂O₃) nanoparticles, gadolinium oxide (Gd₂O₃) nanoparticles, terbiumoxide (Tb_(x)O_(x)) nanoparticles, dysprosium oxide (Dy₂O₃)nanoparticles, holmium oxide (Ho₂O₃) nanoparticles, erbium oxide (Er₂O₃)nanoparticles, thulium oxide (Tm₂O₃) nanoparticles, ytterbium oxide(Yb₂O₃) nanoparticles, and lutetium oxide (Lu₂O₃) nanoparticles.

The metal nanoparticles may comprise one or more nanoparticles selectedfrom the group consisting of lithium nanoparticles, sodiumnanoparticles, potassium nanoparticles, rubidium nanoparticles, cesiumnanoparticles, francium nanoparticles, beryllium nanoparticles,magnesium nanoparticles, calcium nanoparticles, strontium nanoparticles,barium nanoparticles, radium nanoparticles, aluminum nanoparticles,gallium nanoparticles, indium nanoparticles, tin nanoparticles, thalliumnanoparticles, lead nanoparticles, bismuth nanoparticles, scandiumnanoparticles, titanium nanoparticles, vanadium nanoparticles, chromiumnanoparticles, manganese nanoparticles, iron nanoparticles, cobaltnanoparticles, nickel nanoparticles, copper nanoparticles, zincnanoparticles, yttrium nanoparticles, zirconium nanoparticles, niobiumnanoparticles, molybdenum nanoparticles, technetium nanoparticles,ruthenium nanoparticles, rhodium nanoparticles, palladium nanoparticles,silver nanoparticles, cadmium nanoparticles, lanthanum nanoparticles,hafnium nanoparticles, tantalum nanoparticles, tungsten nanoparticles,rhenium nanoparticles, osmium nanoparticles, iridium nanoparticles,platinum nanoparticles, gold nanoparticles, mercury nanoparticles, andcombinations or alloys thereof.

In disclosed embodiments, the sensors are capable of detecting a targetanalyte within a wide temperature range and/or within a wide humidityrange. In addition, the disclosed sensors are capable of detecting atarget analyte at a temperature of less than about 100° C. In someimplementations, the sensors are capable of detecting a target analyteat room temperature (e.g., between about 18° C. and about 24° C.). Insome implementations, the target analyte comprises a gas (e.g., NO₂, H₂,CH₄, CO₂, CO, NH₃, O₂, SO_(x), H₂S, Cl₂, HCl and/or HCN). In someimplementations, the target analyte is an alcohol vapor (e.g., methanol,ethanol, n-propanol, isopropanol, n-butanol, and isobutanol). In otherembodiments, the target analyte is a volatile organic compound (VOC)(e.g., benzene, toluene, ethylbenzene, xylene, chlorobenzene,formaldehyde benzene, formaldehyde, methanol, ethanol, isopropanol,hexane, acetone, tetrachloroethylene (TCE), MTBE, methylene chloride,d-limonene, methylene chloride, an alkane (e.g., propane, butane), aketone, a silane, a siloxane, and mixtures such as diesel/gasolinevapor). In other embodiments, the target analyte comprises a chemicalwarfare agent (CWA) (e.g., tabun (GA), sarin (GB), soman (GD),cyclosarin (GF), sulfur mustard (HD), nitrogen mustard (HN)), or anothersimulant chemical (e.g., Dimethyl methylphosphonate (DMMP), Triethylphosphonate (TEP)).

The disclosed sensors are capable of detecting the target analyte at aconcentration of less than about 1%. In some implementations, theconcentration of the target analyte detected is between about 1 partsper million and about 50 parts per billion. In addition, the disclosedsensors are capable of detecting humidity from between about 0% to about100% relative humidity (RH). In disclosed embodiments, the sensingdevice has a response and recovery time of less than about 180 seconds,preferably less than about 75 seconds.

The present invention is also directed to a nanostructure sensing devicecomprising a semiconductor nanostructure having an outer surface, and atleast one of metal or metal-oxide nanoparticle clusters functionalizingthe outer surface of the nanostructure. The dimensions and overalllength of the sensing device may vary (e.g., from nanometer tocentimeter scale). In some embodiments, the sensing device comprises achip-scale package having dimensions of less than about 10 mm by 10 mm,e.g., about 4 mm by 4 mm or smaller. Thus, the sensor devices of thepresent invention are suitable for wearables and/or compactapplications, and also reliable and robust and thus suitable forindustrial applications and/or extreme conditions. The architecture mayprovide for a single sensing device or multiple sensing devices, e.g.connected in series or parallel. The resulting structure forms aphotoconductive nanostructure/nanocluster hybrid sensor enablinglight-assisted sensing of a target analyte.

In disclosed embodiments, the sensing device exhibits a change incurrent, resistivity and/or voltage upon exposure to a target analyte inthe presence of UV light and/or visible light, with the magnitude ofsuch change dependent on the configuration of the device. Thearchitecture may provide for a single sensor, dual sensors or multiplesensors on a single chip (e.g., comprising nano-, micro-, or millimetersize wide-band gap oxide semiconductors) which are functionalized withmetal oxide and/or metal nanoparticles and/or with their alloys. Thearchitecture is thus capable of simultaneously detecting one, two, ormultiple (e.g., 8 or more) analytes independently. The target analyte(s)interacts with the functionalized material in the presence of variouscarrier gases (e.g., air, oxygen, nitrogen, argon) that results in achange in current, resistance and/or voltage in the semiconductorbackbone. For comparison, a chip consisting of a non-functionalized,passivated element mimicking the design architecture of the sensorelement which is buried under an oxide acts as a baseline, and does notrespond to any of the target analytes.

The semiconductor backbone may comprise etched/patterned ultra-thinnano-clustered metal oxide, e.g. comprised of zinc oxide (ZnO), titaniumdioxide (TiO₂), tin oxide (SnO₂), nickel oxide (NiO), copper oxide,cobalt oxide, iron oxide, zinc magnesium oxide, magnesium oxide,vanadium oxide, lanthanum oxide, and/or zirconium oxide. The secondmetal oxide functionalization consists of another metal oxide, e.g.comprised of zinc oxide (ZnO), titanium dioxide (TiO₂), tin oxide(SnO₂), nickel oxide (NiO), copper oxide, cobalt oxide, iron oxide, zincmagnesium oxide, magnesium oxide, vanadium oxide, lanthanum oxide,and/or zirconium oxide.

Third metal nanoparticles may also be provided, e.g. comprised oflithium, sodium, potassium, rubidium, cesium, francium, beryllium,magnesium, calcium, strontium, barium, radium, aluminum, gallium,indium, tin, thallium, lead, bismuth, scandium, titanium, vanadium,chromium, manganese, iron, cobalt, nickel, copper, zinc, yttrium,zirconium, niobium, molybdenum, technetium, ruthenium, rhodium,palladium, silver, cadmium, lanthanum, hafnium, tantalum, tungsten,rhenium, osmium, iridium, platinum, gold, mercury, and combinations andalloys thereof.

In some implementations, the nanoparticle clusters are multicomponentclusters comprising first metal-oxide nanoparticles and second metalnanoparticles. The nanostructure has a first bandgap, and thenanoparticle clusters have a second bandgap equal to or less than saidfirst bandgap. In disclosed embodiments, the devices exhibit increasedconductivity upon exposure to the target analyte in the presence ofradiation, including UV light and/or visible light.

BRIEF DESCRIPTION OF THE DRAWINGS

The patent or application file contains at least one drawing/photographexecuted in color. Copies of this patent or patent applicationpublication with color drawing(s) will be provided by the Office uponrequest and payment of the necessary fee.

FIG. 1, plates (a) and (b), are schematic representations of a GaN(Nanowire)-TiO₂ (Nanocluster) hybrid sensor according to the presentinvention. FIG. 1, plate (a) shows the sensor in the dark showingsurface depletion of the GaN nanowire, and FIG. 1, plate (b) shows thesensor under UV excitation with photodesorption of O₂ due to holecapture.

FIG. 2, plate (a), illustrates graphically the photoresponse of a hybriddevice (diameter 300 nm) to 1000 ppm of benzene and toluene mixed in airand nitrogen. FIG. 2, plate (b) illustrates the response of a hybriddevice (diameter 500 nm) to different concentrations of water in air.

FIG. 3 is a schematic representation of depletion in the TiO₂ NC in thepresence of oxygen and water, and its effect on the photogeneratedcharge carrier separation in GaN NW. Circles in valence band indicateholes and circles in conduction band indicate electrons.

FIG. 4 illustrates graphically the photo-response of the GaN/(TiO₂—Pt)device to 1000 μmol/mol of ethanol in air and nitrogen, and to 1000μmol/mol of water in air. The devices did not respond to water innitrogen. The air-gas mixture was turned on at 0 s and turned off at 100s.

FIG. 5, plate (a) illustrates graphically UV photo-response of theGaN/(TiO₂—Pt) hybrid device to 1000 μmol/mol (ppm) of methanol, ethanol,and water in air, and hydrogen in nitrogen. The air-gas mixture wasturned on at 0 s and turned off at 100 s. FIG. 5, plate (b) illustratesthe cyclic response of the GaN/(TiO₂—Pt) hybrid device when exposed to2500 μmol/mol (ppm) of hydrogen in nitrogen. The bias voltage for allthe devices was 5 V.

FIG. 6, plate (a) is a scanning electron microscope (SEM) image of theNW bridge structure according to the present invention. FIG. 6, plate(b) shows ZnO nanoparticles on the facets of GaN NW. FIG. 6, plate (c)illustrates graphically current-voltage (I-V) characteristics of thedevice before and after rapid thermal anneal (RTA). FIG. 6, plate (d) isan x-ray diffraction (XRD) Ω—2Θ scan of a 300-nm-thick ZnO film.

FIG. 7 illustrates graphically device response to 500-μmol/mol (ppm) ofmethanol. The inset graph at the bottom left shows the sensitivity oftwo devices toward 500 μmol/mol (ppm) of each isomer of butanol (withDevice 1 shown as the right bar above each isomer, and Device 2 shown asthe left bar above each isomer). The inset graph at the bottom rightshows the response to ethanol, acetone, benzene, and hexane. Sensitivity(S) is given by (I_(g)—I_(a))×100/I_(a), where I_(g) is the devicecurrent in the presence of an analyte in breathing air and I_(a) is thecurrent in pure breathing air, both measured 300 s after the flow isturned on. Percentage standard deviation of the device sensitivity is3.2% based on the five data points collected over a period of 3 days inresponse to the breathing air.

FIG. 8 illustrates graphically device response to different flow ratesof breathing air (plate (a)) and nitrogen gas (plate (b)). The flowrates of the gas are denoted as a=20 sccm, b=40 sccm, c=60 sccm, d=80sccm, and e=100 sccm.

FIG. 9, plate (a) is a schematic illustration of a nanostructuredsemiconductor-nanocluster hybrid gas sensor according to an embodimentof the present invention. The sensor works with low-intensity light froman LED. The emission wavelength is determined by the semiconductor andmetal-oxide bandgaps. FIG. 9, plate (b) illustrates schematically anexemplary thin-film device including a semiconductor backbonefunctionalized with TiO₂ on a sapphire substrate. The smoothness of thesubstrate and film after thermal processing is shown in FIG. 9, plates(c) and (d).

FIG. 10 is a schematic illustration of the mechanism of sensing usingthe disclosed nanocluster-functionalized semiconductor devices. Thesensing is due to the effective separation of photogenerated chargecarriers in the semiconductor backbone caused by surface potentialmodification of the backbone by the nanocluster upon adsorption ofchemicals. The light produces electron-hole pairs in the semiconductor,and also surface defects on the cluster due to photo desorption ofoxygen and water.

FIG. 11 illustrates schematically the epitaxial layer structure utilizedin sensor device fabrication according to an embodiment of theinvention.

FIG. 12 illustrates schematically sensor designs according to thepresent invention, including a sensor having serial architecture (plate(a)), and a sensor having parallel architecture (plate (b)).

FIG. 13 are schematic illustrations of a series architecture design of asensor with four segments, including a top view (plate (a)) and across-section view taken along the dashed line (plate (b)). The sensoroutput is the voltage between the +V_(sensor) and ground pads. TheV_(cal) are the real-time calibration probes for baseline andtemperature drift compensating.

FIG. 14 illustrates graphically a generic sensor calibration curve.Sensitivity S is defined as the slope of the sensor output response vs.analyte concentration plot. The sensor output may be a change incurrent, voltage, or resistance.

FIG. 15 is a schematic illustration of photoexcitation of both themetal-oxide cluster and the GaN backbone using 365 nm light.

FIG. 16 is a schematic illustration showing selectivity tuning using amulticomponent design of nanoclusters. As shown, the target analyte isNO₂ and the interfering chemical is CO₂.

FIG. 17 illustrates graphically depletion depth induced by Ptnanoclusters on GaN and TiO₂ (as calculated by Equation (12) below).

FIG. 18 is a schematic illustration of an integration scheme forstandalone system, showing components at roughly their actual size.

FIG. 19 is a schematic illustration of a hybrid sensor fabricationprocess according to the present invention.

FIG. 20, plates (a-c), are field-emission scanning electron microscopy(FESEM) images of three different sputtered thickness of TiO₂ coatings:including 2 nm (plate (a)), 5 nm (plate (b)), and 8 nm (plate (c)) ofTiO₂ sputtered on GaN nanowires.

FIG. 21 illustrates graphically an XRD Ω−2Θ scan of 150 nm thick TiO₂film deposited on SiO₂/Si substrate at 300° C. and annealed at 650° C.for 45 s in RTA. All indices correspond to the anatase phase[PDF#84-1285].

FIG. 22 illustrates typical morphologies of a 20 nm thick TiO₂ filmsputtered on n-GaN nanowires and annealed at 700° C. for 30 s. FIG. 22,plate (a) is a TEM image showing non-uniformly distributed 2 nm to 10 nmdiameter individual TiO₂ particles, with some of the particles marked bywhite circles. FIG. 22, plate (b) is a high-resolution transmissionelectron microscopy (HRTEM) image of an edge of the GaN nanowire withthe sputtered TiO₂ film. The FFT pattern from the boxed area is shown inexploded view in the upper left inset, indicating 0.35 nm latticefringes which are consistent with a (101) reflecting plane of anatase.

FIG. 23, plate (a) is a BF-STEM image with 5 nm to 10 nm TiO₂nanoparticles barely visible near an edge of a GaN nanowire, with someof the nanoparticles marked by circles. FIG. 23, plate (b) is anADF-STEM image of a TiO₂-containing aggregate on the edge of a GaNnanowire. FIG. 23, plate (c) is an X-ray spectrum of an individual 5 nmTiO₂ particle shown by circled portion ‘A’ in plate (a). FIG. 23, plate(d) is an EEL spectra recorded on position 1 (tip of the aggregate) andposition 2 (edge of the GaN nanowire), as identified in plate (b),respectively.

FIG. 24 illustrates I-V characteristics of a GaN NW two-terminal devicein the dark at different stages of processing. The inset shows thenanowire device with length 5.35 μm and diameter 380 nm. The scale baris 4 μm. The thickness of sputtered TiO₂ film was 8 nm.

FIG. 25, plate (a) illustrates graphically the dynamic photocurrent of abare GaN NW. FIG. 25, plate (b) illustrates the dynamic photocurrent ofa TiO₂ coated (8 nm deposit) GaN NW. The diameters of both nanowireswere about 200 nm. The applied bias is 5 V in both cases.

FIG. 26 illustrates graphically the dynamic response of a singleGaN—TiO₂ hybrid device to 1000 ppm of toluene. For each cycle, the gasexposure time was 100 s. The inset shows the nanowire device with 8.0 μmlength and diameter 500 nm. The scale bar is 5 μm.

FIG. 27, plate (a) illustrates the response of a singlenanowire-nanocluster hybrid sensor (inset shows nanowire with diameter500 nm) to 1000 ppm benzene, toluene, ethylbenzene, chlorobenzene, andxylene in presence of UV excitation. FIG. 27, plate (b) illustrates theresponse of a different sensor (inset shows nanowire with diameter 300nm) to 200 ppb toluene, benzene, ethylbenzene, and xylene with UVexcitation. The total flow in to the chamber was kept constant at 20sccm. The response to air is also shown. The scale bars are 5 μm.

FIG. 28 illustrates graphically a hybrid sensor's photoresponsecharacteristics: FIG. 28, plate (a) shows the characteristics of thedevice shown in FIG. 27, plate (a) for 100 to 10000 ppm concentrationrange of toluene; FIG. 28, plate (b) shows the characteristics of thedevice shown in FIG. 27, plate (b) for 50 ppb to 1 ppm concentrationrange of toluene.

FIG. 29 illustrates sensitivity plots of a GaN—TiO₂ nanowire-nanoclusterhybrid device (diameter 300 nm) for benzene, toluene, ethylbenzene,chlorobenzene, and xylene. The plot identifies the sensor's ability tomeasure wide range of concentration of the indicated chemicals.

FIG. 30 is an HRTEM image of a GaN NW with TiO₂ sputtered on them, withplate (a) showing the GaN NW before Pt and plate (b) showing after Ptdeposition. Circled areas in plate (a) indicate partially aggregatedpolycrystalline TiO₂ particles on the NW surface and on the supportingcarbon film. Arrows in plate (b) in the inset at the upper left mark Ptclusters decorating a 6 nm diameter particle of titanium. The TiO₂particle exhibits 0.35 nm fringes corresponding to (101) lattice spacingof anatase polymorph. 2 nm to 5 nm thick amorphized surface film areindicated by black arrows.

FIG. 31 illustrate an HAADF-STEM of a GaN NW coated with TiO₂ and Pt.,with plate (a) showing 1 nm to 5 nm bright Pt nanoparticles (shown byarrows) decorating surfaces of a polycrystalline TiO₂ island-like filmand of a GaN nanowire. Medium grey aggregated TiO₂ particles (outlinedby dashed line in plate (a)) are barely visible on a thin carbon supportnear the edge of the nanowire. Plate (b) is a high magnification imageof the supporting film near the edge of the nanowire exhibiting 0.23 nmto 0.25 nm (111) and 0.20 nm to 0.22 nm (200) fcc lattice fringesbelonging to Pt nanocrystallites, with arrows indicating amorphous-likePt clusters of 1 nm and less in diameter.

FIG. 32 illustrates I-V characteristics of the hybrid sensor device atdifferent stages of processing. FIG. 32, plate (a) shows GaN/(TiO₂—Pt)hybrids; FIG. 32, plate (b) shows GaN/Pt hybrids. The inset image inplate (b) shows the plan-view SEM image of a typical GaN NWNC hybridsensor. The scale bar in the inset is 4 μm.

FIG. 33 illustrates graphically depletion depth induced by Pt NCs on GaNand TiO₂ as calculated by equation 12.

FIG. 34 illustrates comparative sensing behavior of the three hybridsfor 1000 μmol/mol (ppm) of analyte in air: light gray bar graphs(benzene, toluene, ethylbenzene, xylene, chlorobenzene) representGaN/TiO₂ hybrids, patterned bar graphs (ethanol, methanol, and hydrogen)represent GaN/(TiO₂—Pt) hybrids, and white bar graph (hydrogen)represents GaN/Pt hybrids. Other chemicals which did not produce anyresponse in any one of the hybrids are not included in the plot. Thezero line is the baseline response to 20 sccm of air and N₂. For thisplot the magnitude of the sensitivity is used. The error bars representthe standard deviation of the mean sensitivity values for every chemicalcomputed for 5 devices with diameters in the range of 200 nm-300 nm.

FIG. 35, plate (a) illustrates graphically the photo-response ofGaN/(TiO₂—Pt) hybrid device to different concentrations of methanol inair. FIG. 35, plate (b) shows photo-response of the same device todifferent concentrations of hydrogen in nitrogen. The air-gas mixturewas turned on at 0 s and turned off at 100 s.

FIG. 36, plate (a) is a sensitivity plot of the GaN/(TiO₂—Pt) hybriddevice to ethanol, methanol, and water in air and to hydrogen innitrogen ambient. FIG. 36, plate (b) shows graphically a comparison ofthe sensitivity of GaN/(TiO₂—Pt) and GaN/Pt devices to differentconcentrations of hydrogen in nitrogen.

FIG. 37 illustrates schematically an exemplary fabrication flow chartfor semiconductor-nanocluster based gas sensors according to the presentinvention.

FIG. 38, plate (a) is an image of large area etched nanostructures ofGaN on silicon and sapphire substrate formed according to disclosedprocesses such as shown in FIG. 37. FIG. 38, plate (b) shows an image ofa nanostructure of GaN on silicon and sapphire using ICP etching andpost-etching surface treatment. This nanostructure forms the backbone ofthe disclosed sensors in disclosed embodiments.

FIG. 39 is an RTEM image of a GaN NW with TiO₂ sputtered on them.Circled portions indicate partially aggregated polycrystalline TiO₂particles on the NW surface and on the supporting carbon film.

FIG. 40 illustrates graphically I-V characteristics of a GaN NWtwo-terminal device at different stages of processing.

FIG. 41, plate (a) illustrates graphically response of a single,nanowire-nanocluster hybrid sensor to 100 ppb of benzene, toluene,nitrobenzene, nitrotoluene, dinitrobenzene, dinitrotoluene andtrinitrotoluene in the presence of UV excitation. FIG. 41, plate (b)shows the response of the device to different concentrations oftrinitrotoluene.

FIG. 42 is a sensitivity plot of a GaN—TiO₂ nanowire-nanocluster hybriddevice for benzene, toluene, nitrotoluene, nitrobenzene, DNT, DNB andTNT.

FIG. 43 illustrates sensitivity of two different nanowire-nanoclusterhybrid sensors to 100 ppb of the different aromatic compounds.

FIG. 44, plate (a), illustrates the dynamic responses of a TiO₂ basedsensor exposed to 250 ppm NO₂ mixed with breathing air under UVillumination and dark at room temperature. Plate (b) illustrates theresponse under UV at mixture of 100 ppm, 250 ppm, and 500 ppm withbreathing air. The inset in plate (b) shows the measured responses underUV as a function of NO₂ concentrations with uncertainty. Sensitivity Sis presented by (I_(g)−I_(a))×100/I_(a), wherein I_(g) is the devicecurrent in the presence of an analyte in breathing air and I_(a) is thecurrent in pure breathing air, both measured 300 s after the flow isturned on.

FIG. 45 illustrates schematically an NO₂ gas sensing mechanism of theTiO₂ sensor under UV illumination: plate (a) shows the mechanism in adark environment with breathing air in; plate (b) shows the mechanismunder UV illumination in breathing air; and plate (c) shows themechanism under UV illumination with mixture of NO₂ and breathing air(all at room temperature).

FIG. 46 illustrates graphically the dynamic response of the TiO₂ basedsensor exposed to 500 ppm NO₂ under UV illumination and under dark atroom temperature.

FIG. 47, plate (a) illustrates a GIXRD scan of thermally processedultrathin TiO₂ film, and plate (b) illustrates optical properties(bandgap).

FIG. 48, plate (a) illustrates schematically a SnO₂—Cu nanocluster CO₂sensor. Plates (b) and (c) are AFM images of the SnO₂—Cu nanocluster CO₂sensor.

FIG. 49 illustrates the dynamic responses of the SnO₂—Cu based sensorexposed to CO₂ at room temperature at concentrations of 1000 ppm and5000 ppm. For each cycle, the gas exposure time was 300 s.

FIG. 50 illustrates graphically the response of the SnO₂ based sensor atdifferent relative humidity (RH) concentrations at room temperature.

FIG. 51, plate (a) illustrates the dynamic response of a TiO₂ basedsensor exposed to methanol at room temperature and at a concentration of500 ppm. Plate (b) illustrates the dynamic response of a ZnO basedsensor exposed to benzene at room temperature and a concentration of 500ppm. Plate (c) illustrates the dynamic response of the ZnO based sensorexposed to hexane at room temperature and a concentration of 100 ppm.

FIG. 52 illustrates graphically the dynamic responses of a ZnO—Pd—Agbased sensor exposed to H₂ at room temperature.

FIG. 53 illustrates schematically an exemplary layout of on chipelements of a sensor device in accordance with the present invention.

FIG. 54 illustrates schematically a micro-heater embedded into a sensordevice in accordance with disclosed embodiments of the presentinvention.

FIG. 55 illustrates the temperature profile of 50 μm microheater madefrom a Ti/Ni metal stack MH recorded at 5 V bias voltage (plate (a)) and10 V bias voltage (plate (b)).

FIG. 56 illustrates graphically the dynamic response of a functionalizedGaN sensor device for sensing H₂S (concentration 50 ppm) in dry air.

FIG. 57 illustrates graphically the dynamic response of a functionalizedGaN sensor device for sensing NO₂ (concentration 500 ppm) in dry air(22° C., relative humidity 0-5%).

FIG. 58 illustrates graphically the dynamic response of a functionalizedGaN sensor device for sensing SO₂ (concentration 10 ppm) in dry air (22°C., relative humidity 0-5%).

FIG. 59 illustrates graphically the dynamic response of a functionalizedGaN sensor device for sensing CO₂ (concentration 5000 ppm) in dry air(22° C., relative humidity 0-5%).

FIG. 60 illustrates an exemplary sensor module in accordance with thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed to sensor devices including asemiconductor nanostructure, such as a micro or nanodevice, or nanowire(NW), having a surface functionalized or decorated with metal ormetal-oxide nanoparticles or nanoclusters. When metal/metal-oxidenanoparticles selected according to the disclosed methods are placed onthe surface of a nanostructure, significant changes result in thephysical properties of the system. The nanoparticles increase theadsorption of chemical species by introducing additional adsorptionsites, thereby increasing the sensitivity of the resulting system.

The metal or metal-oxide nanoparticles may be selected to act ascatalysts designed to lower the activation energy of a specificreaction, which produces active radicals by dissociating the adsorbedspecies. These radicals can then spill-over to a semiconductor structure(see Sermon P A and Bond G C (1973) “Hydrogen Spillover,” Catal. Rev.8(2):211-239; Conner W C et al. (1986) “Spillover of sorbed species,”Adv. Catal. 34:1), where they are more effective in charge carriertransfer. Further, the selected nanoparticles modulate the currentthrough the nanowire through formation of nanosized depletion regions,which is in turn a function of the adsorption on the nanoparticles.Nanoparticles or nanoclusters suitable for the present invention includevirtually any metal-oxide and/or metal. Thus, it should be understoodthat the present invention is not limited to the particular exemplarymetal-oxides and/or metals disclosed in the various embodiments andexamples herein.

According to one embodiment, nanowire-nanocluster hybrid chemicalsensors were realized by functionalizing n-type (Si doped) galliumnitride (GaN) NWs with TiO₂ nanoclusters. The sensors selectively sensebenzene and related aromatic environmental pollutants, such as toluene,ethylbenzene, and xylene (sometimes referred to as BTEX). GaN is awide-bandgap semiconductor (3.4 eV), with unique properties (Morkog H(1999) Nitride Semiconductors and Devices, Springer series in MaterialsScience, Vol. 32, Springer, Berlin). Its chemical inertness andcapability of operating in extreme environments (high-temperatures,presence of radiation, extreme pH levels) is thus suitable for thedisclosed sensor design. TiO₂ is a photocatalytic semiconductor with abandgap energy of 3.2 eV (anatase phase). Photocatalytic oxidation ofvarious organic contaminants over titanium dioxide (TiO₂) has beenpreviously studied (see Mills A and Hunte S L (1997) “An overview of ofsemiconductor photocatalysis,” J. Photochem. Photobiol. A 108:1-35; LuoY and Ollis D F (1996) “Heterogeneous photocatalytic oxidation oftrichloroethylene and toluene mixtures in air: Kinetic promotion andinhibition, time-dependent catalyst activity,” J. Catal. 163:1-11). TheTiO₂ nanoclusters were thus selected to act as nanocatalysts to increasethe sensitivity, lower the detection time, and enable the selectivity ofthe structures to be tailored to organic analytes.

The hybrid sensor devices may be developed by fabricating two-terminaldevices using individual GaN NWs followed by the deposition of TiO₂nanoclusters using radio frequency (RF) magnetron sputtering. The sensorfabrication process employed standard micro-fabrication techniques.X-ray diffraction (XRD) and high-resolution analytical transmissionelectron microscopy using energy-dispersive X-ray and electronenergy-loss spectroscopies confirmed the presence of anatase phase inTiO₂ clusters after post-deposition anneal at 700° C.

A change of current was observed for these hybrid sensors when exposedto the vapors of aromatic compounds (e.g., benzene, toluene,ethylbenzene, xylene, and chlorobenzene mixed with air) under UVexcitation, while they had minimal or no response to non-aromaticorganic compounds such as methanol, ethanol, isopropanol, chloroform,acetone, and 1, 3-hexadiene. The sensitivity range for the notedaromatic compounds, except chlorobenzene, were from about 1% down toabout 50 parts per billion (ppb) at room-temperature. By combining theenhanced catalytic properties of the TiO₂ nanoclusters with thesensitive transduction capability of the nanowires, an ultra-sensitiveand selective chemical sensing architecture is achieved.

As discussed in further detail in Example 1 below, GaN—TiO₂(nanowire-nanocluster) hybrid sensors demonstrated a response tospecific volatile organic compounds mixed with air at ambienttemperature and humidity. In the presence of UV light (e.g., having awavelength in the range of about 10 nm to about 400 nm), these hybridsensor devices exhibited change in the photocurrent when exposed tobenzene, toluene, ethylbenzene, xylene, and chlorobenzene mixed in air.However, gases like methanol, ethanol, isopropanol, chloroform, acetone,and 1, 3-hexadiene exhibited little or no change in the electricalcharacteristics of the devices, thus demonstrating the selectiveresponse of these sensors to the aromatic compounds. Benzene, toluene,ethylbenzene, and xylene were detected by the disclosed sensors at aconcentration level as low as 50 ppb in air. In addition, the disclosedsensor devices are highly stable and able to sense aromatic compounds inair reliably for a wide range of concentrations (e.g., 50 ppb to 1%).

In addition, the disclosed sensors demonstrated highly sensitive andselective detection of traces of nitro-aromatic explosive compounds. Asdiscussed in further detail in Example 5 below, GaN/TiO₂nanowire-nanocluster hybrid sensors detected different aromatic andnitroaromatic compounds at room temperature. For example, the GaN/TiO₂hybrids were able to detect trinitrotoluene (TNT) concentrations as lowas 500 μmol/mol (ppt) in air and dinitrobenzene concentrations as low as10 nmol/mol (ppb) in air in approximately 30 seconds. The notedsensitivity range of the devices for TNT was from 8 ppm down to as lowas 500 ppt. The detection limit of dinitrotoluene, nitrobenzene,nitrotoluene, toluene and benzene in air is about 100 ppb with aresponse time of ≅75 seconds. Devices according to the present inventionexhibited sensitive and selective response to TNT when compared tointerfering compounds like toluene. Thus, the disclosed sensors aresuitable for use as highly sensitive, selective, low-power and smartexplosive detectors, which are relatively inexpensive to manufacture inlarger quantities.

Based on structural analysis, an exemplary mechanism that qualitativelyexplains the hybrid sensor's response to different analytes is shown inFIG. 1. With regard to the photocatalytic processes on the TiO₂ surface,the oxygen vacancy defects (Ti³⁺ sites) on the surface of TiO₂ are theactive sites responsible for adsorption of species like oxygen, water,and organic molecules (see Yates Jr J T (2009) “Photochemistry on TiO2:mechanisms behind the surface chemistry,” Surf. Sci. 603:1605-1612).Interestingly, a relatively defect free TiO₂ surface, generated byannealing in high-oxygen flux, is chemically inactive (Li M et al.(1999) “Oxygen-induced restructuring of rutile TiO ₂(110): formationmechanism, atomic models, and influence on surface chemistry,” FaradayDiscuss. 114:245). Experimental studies and simulations reveal thatmolecular oxygen is chemisorbed on the surface vacancies (Ti³⁺ sites),acquiring a negative charge as shown in FIG. 1, plate (a) (Anpo M et al.(1999) “Generation of superoxide ions at oxide surfaces,” Top. Catal.8:189-198; de Lara-Castells M P and Krause J L (2003) “Theoretical studyof the UV-induced desorption of molecular oxygen from the reduced TiO ₂(110) surface,” J. Chem. Phys. 118:5098). This is due to the presence ofthe localized electron density at or near exposed Ti³⁺ atoms on the TiO₂surface (Henderson M A et al. (1999) “Interaction of Molecular Oxygenwith the Vacuum-Annealed TiO2(110) Surface: Molecular and DissociativeChannels,” J. Phys. Chem. B 103:5328-5337). Water may also be present onthe TiO₂ cluster surface via molecular or dissociative adsorption,producing OH⁻ species on the defect sites (Lee F K et al. (2007) “Roleof water adsorption in photoinduced superhydrophilicity on TiO ₂ thinfilms,” Appl. Phys. Lett. 90:181928; Bikondoa O et al. (2006) “Directvisualization of defect-mediated dissociation of water on TiO ₂ (110),”Nat. Mater. 5:189-192).

Although most of the theoretical and experimental studies on oxygen andwater adsorption are done for the (110) surface of rutile phase, thereare studies that suggest that similar adsorption behavior is alsoexpected for the anatase surface (Wahab H S et al. (2008) “Computationalinvestigation of water and oxygen adsorption on the anatase TiO ₂ (100)surface,” J. Mol. Chem. Struct. 868:101-108). The GaN NW has a surfacedepletion region as shown in FIG. 1, plate (a), which determines itsdark conductivity (Sanford N A et al. (2010) “Steady-state and transientphotoconductivity in c-axis GaN nanowires grown bynitrogen-plasma-assisted molecular beam epitaxy,” J. Appl. Phy.107:034318).

In the presence of UV excitation with an energy above the bandgap energyof anatase TiO₂ (3.2 eV) and GaN (3.4 eV), electron-hole pairs aregenerated both in the GaN NW and in the TiO₂ cluster, as shown in FIG.1, plate (b). Photogenerated holes in the nanowire tend to diffusetowards the surface due to the surface band bending. This effect ofseparation of photogenerated charge carriers results in a longerlifetime of photogenerated electrons, which in turn enhances thephotoresponse of the nanowire devices. On the TiO₂ cluster surface,however, the photogenerated charge carriers lead to a differentphenomenon. In n-type semiconductor oxides such as TiO₂, the surfaceadsorption produces upward band-bending, which drives the photogeneratedholes towards the surface. The chemisorbed oxygen molecule (O₂ ⁻) andhydroxide ions (OH⁻) can readily capture a hole and desorb as shown inFIG. 1, plate (b) (Perkins C L and Henderson M A (2001) “Photodesorptionand Trapping of Molecular Oxygen at the TiO ₂(110)—Water Ice Interface,”J. Phys. Chem. B. 105:3856-3863; Thompson T L and Yates J T Jr. (2006)“Control of a surface photochemical process by fractal electrontransport across the surface: O(2) photodesorption from TiO(2)(110),” J.Phys. Chem. B 110:7431-7435). The decrease of photocurrent through thesehybrid sensors when exposed to 20 sscm of air may be due to the increasein oxygen concentration at the surface of TiO₂ clusters, leading to anincrease in trapping of photogenerated holes at the surface. Thisprocess results in increased lifetime of photogenerated electrons. Asthese nanowires are n-type, excess negative charge on the surface of thewire (on the TiO₂ clusters) reduces the nanowire current, thus providinga local-gating effect due to net negative charge accumulation in theTiO₂ clusters. Thus, the photoinduced oxygen desorption and subsequentcapture of holes by organic adsorbate molecules on the surface of TiO₂clusters produces the local-gating effect, which is responsible for thesensing action of the disclosed sensor devices. The adsorbed hydroxylions may also trap a hole forming OH. species. Other effects such asdiffusion of carriers between the clusters and the nanowire may alsohave a role in the sensing properties of the sensors.

Although some embodiments are described in term of excitation in thepresence of UV light, it should be understood that excitation byradiation of other wavelengths may be more suitable for devices havingother types of metal-oxide and/or metal nanoparticles. For example,excitation in the presence of visible light (i.e., having a wavelengthof between about 380 nm and about 740 nm) is suitable for someembodiments.

The process noted above and shown in FIG. 1 also explains sensorresponse when exposed to N₂ flow, as shown in FIG. 2, plate (a). In thepresence of 20 sccm of N₂ flow, the photocurrent in the sensorsincreases significantly in comparison with 20 sccm of air flow. In an N₂environment, oxygen is desorbed from the surface vacancy sites bycapturing photogenerated holes, but does not get re-adsorbed, resultingin significant reduction of hole capture. As such, the photogeneratedelectron-hole pairs recombine effectively in the cluster. Thus, thephotocurrent through the nanowire/nanocluster hybrid sensor, which isotherwise increased due to the local-gating effect by the TiO₂ clusters,is absent in an N₂ environment.

In the presence of water in air, the photocurrent through these sensorsrecovers towards the level without air flow, as seen in FIG. 2, plate(b), indicating a reduction of the hole trapping due to adsorption ofwater on the TiO₂ surface. Water may be adsorbed as a molecule on thedefect sites replacing O₂ (see Herman G S et al. (2003) “ExperimentalInvestigation of the Interaction of Water and Methanol with Anatase-TiO₂(101),” J. Phys. Chem. B 107:2788-2795). With increasing waterconcentration, more defects are filled with water. If the adsorbed waterdissociates and produces OH⁻ species, then it is possible that it willact as hole traps and decrease the photocurrent the same way thephotodesorption of oxygen does. A competition between the molecularwater adsorption (reducing hole capture) and dissociative wateradsorption (increasing hole capture) is possible, with the dominantprocess ultimately determining the photocurrent level in the nanowiresin the presence of water.

The presence of aromatic compounds such as benzene, ethylbenzene,chlorobenzene, and xylene in air reduced the photocurrent (e.g. see FIG.2, plate (a)). Organic molecules are known hole-trapping adsorbates (seeYamakata A et al. (2002) “Electron-and hole-capture reactions on Pt/TiO₂ photocatalyst exposed to methanol vapor studied with time-resolvedinfrared absorption spectroscopy,” J. Phys. Chem. B 106:9122-9125). Mostaromatic compounds show high affinity for electrophilic aromaticsubstitution. The exact mechanism of photooxidation of adsorbed organiccompounds on TiO₂ is complex. However, it is believed that oxidationoccurs by either indirect oxidation via the surface-bound hydroxylradical (i.e., a trapped hole at the TiO₂ surface) or directly via thevalence-band hole before it is trapped either within the particle or atthe particle surface (see Nosaka Y et al. (1998) “Factors governing theinitial process of TiO2 photocatalysis studied by means of in situelectron spin resonance measurements,” J. Phys. Chem. B 102:10279-10283;Mao Y et al. (1991) “Identification of organic acids and otherintermediates in oxidative degradation of chlorinated ethanes on titaniasurfaces en route to mineralization: a combined photocatalytic andradiation chemical study,” J. Phys. Chem. 95:10080-10089). In thepresence of air (with residual water) hydroxyl mediated hole transfer toadsorbates such as benzene, xylene is dominant, whereas in the N₂environment direct transfer of valence band holes to aromatic adsorbatescould be possible.

Irrespective of the hole transfer mechanism, the presence of additionalhole traps reduces the sensor photocurrent, as observed in the presenceof benzene mixed with N₂ and air as shown in FIG. 2, plate (a). Themodel disclosed herein qualitatively explains the observed trends forcompounds tested, such as benzene, ethylbenzene, chlorobenzene, andxylene. However, toluene exhibits a different trend, which may be due toother second order effects other than or in addition to the holetrapping mechanism.

The disclosed mechanism is further validated when comparing ionizationenergies of various compounds tested with the responses generated whenthe sensors are exposed to them (see Table I). The effectiveness of theprocess of hole transfer to the adsorbed organic molecules relates tothe compound's ability to donate an electron (i.e. the lower theionization energy of a compound, the easier for it to donate an electronor capture a hole). The observed sensitivity trend for benzene (lowestsensitivity), ethylbenzene, and xylene (highest sensitivity) correlateswith their ionization energies as shown in Table I, with benzene beingthe highest and xylene the lowest among the three.

TABLE I Physical Properties of Various Compounds Tested Organic CompoundSensitivity Ionization Potential (eV) Chloroform No 11.37 Ethanol No10.62 Isopropanol No 10.16 Cyclohexane Yes 9.98 Acetone No 9.69 BenzeneYes (Min) 9.25 Chlorobenzene Yes 9.07 Toluene Yes 8.82 Ethylbenzene Yes8.77 Xylene Yes (Max) 8.52 1,3-Hexadiene No 8.50

As shown in Table I, the sensitivity trend is consistent for aromatics,given 1,3-Hexadiene produced no response in the sensors. Although mostfunctional groups with either a non-bonded lone pair or p-conjugationshow oxidative reactivity towards TiO₂ (Hoffman M R et al. (1995)“Environmental Applications of Semiconductor Photocatalysis,” Chem. Rev.95:69-96), aromatic compounds are more easily photocatalyzed thanaliphatic ones under the same conditions (Carp O et al. (2004)“Photoinduced reactivity of titanium dioxide,” Prog. Solid St. Chem.32:33-177).

Thus, the metal-oxide nanoclusters (TiO₂) on GaN NWs or nanostructuresdemonstrate the disclosed architecture for highly selective gas sensing.The exemplary sensors are capable of selectively sensing benzene andrelated aromatic compounds at nmol/mol (ppb) level in air atroom-temperature under UV excitation.

According to another embodiment, the specific selectivity of thedisclosed nanowire (or nanostructure)/nanocluster hybrid sensors may betailored using a multi-component nanocluster design. For example,catalytic metals (e.g., platinum (Pt), palladium (Pd), and/or any othertransition metals) are deposited onto the surface of oxidephotocatalysts in order to enhance their catalytic activity. Metalclusters on a metal-oxide catalyst alter the behavior of the metal-oxidecatalyst by any one, or a combination of, the following mechanisms: 1)changing the surface adsorption behavior as metals often have verydifferent heat of adsorption values compared to the metal-oxides; 2)enabling catalytic decomposition of certain analytes on the metalsurface, which otherwise would not be possible on the oxide surface; 3)transporting active species to the metal-oxide support by the spill-overeffect from the metal cluster; 4) generating a higher degree ofinterface states, thus increasing reactive surface area reaction area;5) changing the local electron properties of the metal clusters, such asworkfunction, due to adsorption of gases; and 6) effectively separatingphotogenerated carriers in the underlying metal-oxide. The effect oftransition metal loading such as iron (Fe), copper (Cu), Pt, Pd, andrhodium (Rh) onto TiO₂ has been evaluated for photocatalyticdecomposition of various chemicals in both gas-solid and liquid-solidregimes.

In one implementation, the selectivity of the titanium dioxide (TiO₂)nanocluster-coated gallium nitride (GaN) nanostructure sensor device isaltered by addition of platinum (Pt) nanoclusters. In anotherimplementation, the sensor device includes Pt nanocluster-coated GaNnanostructure. The hybrid sensor devices may be developed by fabricatingtwo-terminal devices using individual GaN NWs or nanostructures followedby the deposition of TiO₂ and/or Pt nanoclusters (NCs) using asputtering technique, as described above.

The sensing characteristics of GaN/(TiO₂—Pt) nanowire-nanocluster (NWNC)hybrids and GaN/(Pt) NWNC hybrids is altered as compared to GaN/TiO₂sensors. The GaN/TiO₂ NWNC hybrids show remarkable selectivity tobenzene and related aromatic compounds with no measurable response forother analytes, as discussed above. However, the addition of Pt NCs toGaN/TiO₂ sensors dramatically alters the sensing behavior, making themsensitive only to methanol, ethanol, and hydrogen, but not to otherchemicals tested, as discussed in further detail in Example 2 below.

The GaN/(TiO₂—Pt) hybrid sensors were able to detect ethanol andmethanol concentrations of 100 nmol/mol (ppb) in air in approximately100 seconds, and hydrogen concentrations from 1 μmol/mol (ppm) to 1% innitrogen in less than 60 seconds. However, GaN/Pt hybrid sensors showedlimited sensitivity only towards hydrogen and not towards any alcohols.All the hybrid sensors are operable at room temperature and arephotomodulated (i.e., responding to analytes only in the presence oflight, e.g., ultra violet (UV) light). The selectivity achieved issignificant from the standpoint of numerous applications requiringroom-temperature sensing, such as hydrogen sensing and sensitive alcoholmonitoring. For example, the dynamic response of an exemplary TiO₂ basedsensor exposed to methanol at room temperature and at a concentration of500 ppm is illustrated in FIG. 51, plate (a). The disclosed sensorstherefore demonstrate tremendous potential for tailoring the selectivityof the hybrid nanosensors for a multitude of environmental andindustrial sensing applications.

A qualitative understanding of the selective sensing mechanism of thedisclosed sensors may be developed by considering how differentmolecules adsorb on the nanocluster surfaces, and determining the rolesof intermediate reactions in the sensitivity of the sensors. While someof the embodiments, examples and explanation describe the invention interms of NWs, it should be understood that other nanostructures ormicrostructures may be utilized. Accordingly, the present invention isnot limited to sensors including NWs.

The Photocurrent in GaN/(TiO₂—Pt) Hybrid Sensors in the Presence of Air,Nitrogen, and Water:

The oxygen vacancy defects (Ti³⁺ sites) on the surface of TiO₂ are the“active sites” for the adsorption of species like oxygen, water, andorganic molecules (Yates Jr J T (2009) “Photochemistry on TiO2:mechanisms behind the surface chemistry,” Surf. Sci. 603:1605-1612;Bikondoa O et al. (2006) “Direct visualization of defect-mediateddissociation of water on TiO ₂(110),” Nat. Mater. 5:189-192). It hasbeen observed that oxygen adsorption on photocatalyst powders such asTiO₂ and ZnO quenches the photoluminescence (PL) intensity, whileadsorption of water produces an enhancement of the PL. Electron-trappingadsorbates, such as oxygen, increase the band-bending of TiO₂, whichfacilitates the separation of photogenerated electron hole pairs in theoxide. Subsequently, the PL intensity is decreased as the photogeneratedcharge carries cannot recombine efficiently. Conversely, in the case ofwater, the band bending is reduced, resulting in an increase in the PLintensity. In explaining the observed behavior of the hybrid sensors,the depletion effect induced by the TiO₂ clusters on GaN NW isconsidered. Considering an inverse relationship, i.e., increase indepletion of the TiO₂ cluster leads to a decrease in the depletion widthin the GaN NW and vice versa, some of the observed sensing behavior isexplained.

As shown in FIG. 3, when oxygen is adsorbed on the TiO₂ NC surface, thedepletion width in the NC increases, leading to a decrease in thedepletion width in the NW. Adsorption of water, nitrogen, and alcoholproduce the reverse effect: they decrease the depletion width of theTiO₂ NC, leading to an increase in the band-bending on the GaN NW.Increased band-bending in the GaN NW results in an effective separationof charge carriers, leading to an increase in photocurrent through theNW. This qualitatively explains the increase in the photocurrent whenthe hybrid sensor is exposed to water mixed with air or with purenitrogen (see FIG. 4). However, the increase in the photocurrent whenexposed to 20 sccm of air flow is not fully explained. Under air flow,more oxygen should adsorb on the NCs, causing an increase in thedepletion width of the cluster. This should have resulted in a decreasein the photocurrent based on our assumption; however, an increase in thephotocurrent is exhibited (FIG. 4) when 20 sccm of air is passed throughthe chamber.

In the absence of UV light, the absorption or desorption of chemicalsfrom the cluster surfaces cannot modulate the dark current through thenanowire. In the dark, the surface depletion layer of the GaN NW isthicker compared to under UV excitation (see Mansfield L M et al. (2009)“GaN nanowire carrier concentration calculated from light and darkresistance measurements,” Journal of Electronic Materials 38:495-504).The minority carrier (hole) concentration is also significantly lower.Thus the NCs are ineffective in modulating the dark current through theNW.

Mechanism of Sensing of Alcohols and Hydrogen by GaN/(TiO₂—Pt) NWNCSensors

Adsorption of alcohols (RCH₂—OH) on the TiO₂ surface leads to theiroxidation (Kim K S and Barteau M A (1989) “Reaction of Methanol on TiO₂,” Surface Science 223:13-32). Although there are various mechanisms ofoxidation of adsorbed alcohols on TiO₂ surface, focus is on theoxidation of alcohols by photogenerated holes. The process is describedby the following reactions:RCH₂—OH(g)

RCH₂—OH(ads)  (Equation 1)RCH₂—OH(ads)+h ⁺(photogenerated hole)⇄RCH₂—OH⁺(ads)  (Equation 2)RCH₂—OH⁺(ads)

RCH—OH.(ads)÷H⁺(ads)  (Equation 3)RCH—OH.(ads)

RCHO(ads)+H⁺(ads)+e ⁻  (Equation 4)where (ads) and (g) represent adsorbed and gas phase species,respectively. For Equation 4 to proceed in the forward directions, theH⁺ species should be removed effectively. It is possible that from TiO₂NCs the H⁺ species can spill-over on to Pt clusters nearby, where theycan be reduced to form H₂:2H⁺(ads)+2e ⁻

H₂(g)  (Equation 5)

As H⁺ reduction and hydrogen-hydrogen recombination is weak on the bareTiO₂ surface (Fujishima A et al. (2008) “TiO ₂ photocatalysis andrelated surface phenomena,” Surf. Sci. Rep. 63:515-582), the rate ofalcohol oxidation to aldehyde might be affected by the H⁺ reduction andhydrogen-hydrogen recombination on the Pt NCs. Adsorption of alcoholsand their subsequent oxidation due to trapping of photogenerated holesleads to a decrease in the band bending of TiO₂ NCs. As shown in FIG. 3,this leads to an increase in the NW photocurrent, which is observed forthe GaN/(TiO₂—Pt) sensors when exposed to methanol and ethanol (FIG. 4).It is likely that the production of H.sub.2 on Pt is the key for sensingalcohols by GaN/(TiO₂—Pt) sensors. Additionally, H₂ on Pt surface candissociate and diffuse to the Pt/TiO₂ interface. Atomic hydrogen isshown to produce an interface dipole layer, which reduces the effectivework-function of Pt (Du X et al. (2002) “A New Highly Selective H2Sensor Based on TiO2/PtO—Pt Dual-Layer Films,” Chem. Mater.14:3953-3957). Effective reduction of Pt workfunction also reduces thedepletion width in TiO₂, which according to the model in FIG. 4, alsoleads to an increase in the photocurrent when these sensors are exposedto alcohols. In the presence of hydrogen in nitrogen, the workfunctionchange of Pt NCs due to hydrogen adsorption is the likely cause for thesensing behavior of these sensor hybrids.

Selectivity of GaN/(TiO₂—Pt), GaN/Pt, and GaN/TiO₂NWNC Hybrid Sensors

A significant finding of the present invention is the change in theselectivity of GaN/TiO₂ hybrid sensors due to the addition of Pt NCs.The observed selectivity behavior of the three hybrids can bequalitatively explained if the heat of adsorption of the analytes onTiO₂ and Pt surfaces is considered as shown in Table II and theirionization energies presented in Table III.

TABLE II Heat of Adsorption for Methanol, Benzene, and Hydrogen on Ptand TiO₂ (Anatase*) Hydrogen Benzene Surface (kJ/mol) Methanol (kJ/mol)(kJ/mol) TiO₂ Negligible 92 64 Pt 100 48 117 *The heat of absorptionvalues for TiO2 rutile surfaces are comparable

TABLE III Ionization Energy of the Analytes (CRC Handbook of Chemistryand Physics, 84th ed.; CRC Press: Boca Raton, FL., 2003): OrganicCompound Ionization Energy (eV) Methanol 10.85 Hydrogen 13.5 Benzene9.25

Referring to Table II, benzene has a higher heat of adsorption on Ptthan on TiO₂. Therefore, benzene will preferentially adsorb on Pt in theTiO₂—Pt cluster. Now, in the absence of Pt, when the benzene is adsorbedon TiO₂ it can interact with the photogenerated charge carriersresulting in the sensing behavior of GaN/TiO₂ devices. However, ifbenzene is adsorbed on Pt (such as in the case of TiO₂—Pt and Ptnanoclusters on GaN) then benzene molecules cannot interact withphotogenerated charge carriers in TiO₂, and therefore are ineffective inproducing any current modulation in the nanowire. Thus, benzene isdetected by GaN/TiO₂ sensor devices, but not by GaN/(TiO₂—Pt) and GaN/Ptsensor devices.

Further, methanol is detected by GaN/(TiO₂—Pt) sensors only, and not byGaN/TiO₂ and GaN/Pt sensors. From Table III, methanol (unlike benzene)effectively adsorbs on TiO₂, whether Pt is present or absent (as theheat of adsorption of methanol is higher on TiO₂ than Pt). It isbelieved that methanol on TiO₂ in the absence of Pt does not participatein photogenerated carrier trapping as efficiently as benzene and otheraromatic compounds on the TiO₂ nanoclusters. Referring to Table III, theionization energy of methanol, hydrogen, and benzene is shown. Theeffectiveness of the process of hole transfer to the adsorbed organicmolecules is related to the compound's ability to donate an electron(i.e. the lower the ionization energy of a compound, the easier for itto donate an electron or capture a hole). However, in the presence of Ptnanoclusters nearby, methanol adsorption on TiO₂ ultimately leads toformation of H⁺ through photo-oxidation of methanol, and eventually H₂,which is the key molecule for sensing of methanol by (TiO₂—Pt) NCs onGaN NW. A similar mechanism applies for ethanol sensing by theGaN/(TiO₂—Pt) hybrids.

Hydrogen is detected by GaN/(TiO₂—Pt) and GaN/Pt hybrids, and not byGaN/TiO₂ NWNC sensors, and GaN/(TiO₂—Pt) sensors have a higher responseto hydrogen than to alcohols. From Table II, hydrogen has negligibleheat of adsorption on TiO₂, thus GaN/TiO₂ devices are not sensitive tohydrogen. However, in the presence of Pt NCs on TiO₂, hydrogen canadsorb on the Pt NCs. Once adsorbed, hydrogen can modify theworkfunction of Pt, resulting in a change in the photocurrent throughthe nanowire. However, this is not the only mechanism, as that wouldimply that GaN/Pt hybrids should be equally sensitive to H₂. It isbelieved that when hydrogen is adsorbed on the TiO₂—Pt NC, it alsoreduces the TiO₂ surface. Thus, in the presence of only Pt on GaN,workfunction modification of Pt solely produces change in thephotocurrent in the NW. However, in the presence of Pt and TiO₂ NCs,hydrogen adsorption leads to the modulation of the photocurrent in GaNNW, through modulation of Pt workfunction together with the change inthe depletion layer of the TiO₂ NCs, resulting in a larger change of thephotocurrent, thus higher sensitivity.

The faster and larger response of GaN/(TiO₂—Pt) towards H₂ compared tothe alcohols (as shown in FIG. 5) is due to the fact that in the case ofalcohols, hydrogen is produced after photo-oxidation of the adsorbedalcohols, which is a two-step process with lower yield. In the case ofH₂ in nitrogen, there is a direct availability of H₂ molecules.

GaN/(TiO₂—Pt) sensors are not sensitive to high carbon-containing (C>2)alcohols such as propanol and butanol. In this regard, it has been shownthat the hydrogen production from the photocatalytic oxidation ofalcohols on TiO₂/Pt surface is related to the polarity of the alcohols(i.e., the higher the polarity of the alcohol the greater the yield ofphotocatalytic hydrogen production) (see Yang Y Z et al. (2006)“Photo-Catalytic Production of Hydrogen Form Ethanol over M/TiO2Catalysts (M=Pd, Pt or Rh),” Applied Catalysis B: Environmental67:217-222). The polarity (Y) is defined as Y=(ε_(s)−1)/(ε_(s)+2), whereε_(s) is the relative permittivity of the solvent. Table IV lists thepolarity of various alcohols tested.

TABLE IV Solvent Polarity of Various Alcohols Solvent Polarity Methanol0.91 Ethanol 0.89 n-Propanol 0.86 i-Propanol 0.85 Butanol 0.84

The relative difficulty of producing hydrogen from highercarbon-containing alcohols (C>2) is believed to be the cause of theGaN/(TiO₂—Pt) sensor's inability to detect alcohols with C greater than2. The sensor's greater response to methanol than ethanol (at least forconcentrations above 500 μmol/mol) is also consistent with thepolarities of the alcohols.

The GaN/(TiO₂—Pt) hybrid sensors are operable at room-temperaturesensing of hydrogen, and thus are suitable for various applications(e.g., industrial production facilities, oil refineries, hydrogenmonitoring in hydrogen-powered vehicles, alcohol monitoring systems forindustrial and law-enforcement purposes, etc.). The disclosed mechanismsand methods may be implemented for achieving other multicomponent NWNCbased sensors. For example, the dynamic response of a ZnO—Pd—Ag basedsensor exposed to H₂ at room temperature is illustrated in FIG. 52.Through combinations of metal and metal-oxides available, a library ofsensors may be produced, each with precisely tuned selectivity, on asingle chip for detecting a wide variety of analytes in many differentenvironments.

Thus, an inactive semiconductor nanostructure (e.g., NW) surface may befunctionalized with selected analyte-specific active metal-oxidenanoparticles. For example, another embodiment of the present inventionprovides for alcohol sensors using gallium nitride (GaN) nanowires (NWs)functionalized with zinc oxide (ZnO) nanoparticles. The dynamic responseof exemplary ZnO based sensors exposed to benzene (concentration 500ppm) and hexane (concentration 100 ppm) at room temperature is shown inFIG. 51, plates (b) and (c). The disclosed sensors operate at roomtemperature, are fully recoverable, and demonstrate a response andrecovery time on the order of 100 seconds or less. The sensing isassisted by ultraviolet (UV) light within the 215-400 nm band and withthe intensity of 375 nW/cm² measured at 365 nm.

As discussed above, the conductivity model of GaN nanostructure iscomprised of a conducting channel surrounded by a surface depletionregion, where modulation in the width of the depletion region induces achange in the conductivity of the NW. Similarly, ZnO nanoparticles havea surface depletion layer, which enhances upon exposure to air due toelectron capture by surface-adsorbed oxygen. When UV light is turned on,the photogenerated holes in ZnO assist in removing the adsorbed oxygen,thus releasing the electrons captured by surface oxygen back into ZnO.The photoinduced excess of electrons in the ZnO nanoparticles promotesphotogenerated charge separation in the GaN nanostructure, therebyresulting in increased conductivity. Conversely, there is a reduction inthe number of free electrons in the ZnO nanoparticles when exposed toair, leading to a reduced conductivity. As seen in FIG. 6, this effectincreases with increasing flow rate of air due to enhanced coverage ofthe device surface with adsorbed oxygen.

The device response to alcohols may be explained by the followinggeneric reaction occurring on the surface of ZnO:2CH₃OH+O⁻ ₂(adsorbed)→2HCHO+2H₂O+e ⁻  (Equation 6)

As shown in FIG. 7, the exposure to alcohol vapors leads to increaseddevice conductivity due to the removal of adsorbed oxygen. In the caseof exposure to N₂, although there is no surface reaction, N₂ assists indesorption of the oxygen, thus restoring the conductivity, as shown inFIG. 8.

The disclosed hybrid GaN nanostructure/ZnO nanoparticle devices aresuitable for UV-assisted alcohol sensing at room temperature. Thesedevices are a suitable candidate for making nanosensor arrays because oftheir tunable selectivity, ability to detect the pbb level of analytes,and fast response and recovery time.

The disclosed hybrid chemiresistive architectures utilizingnanoengineered wide-bandgap semiconductor backbone functionalized withmulticomponent photocatalytic nanoclusters of metal-oxides and/or metalsare particularly suitable for larger scale manufacturing techniques,such as for commercial applications. The sensors operate atroom-temperature via photoenabled sensing. A substantial benefit of thedisclosed sensors is the utilization of all standard microfabricationtechniques, thus resulting in economical, multianalyte single-chipsensor solution. By combining the “designer” adsorption properties ofmulticomponent nanoclusters together with sensitive transductioncapability of nanostructured semiconductor backbones, photoenabled, roomtemperature, ultra-sensitive, and highly selective chemical sensors areachieved.

The sub-micron structures may be formed on an epitaxial thin-film grownon non-conductive/semi-insulating substrate using deep UV lithographyand a combination of plasma etching and wet-etching. Such structures arefunctionalized with multicomponent nanoclusters of metal-oxides andmetals using reactive-sputter deposition, as noted above.

Referring to FIG. 9, an exemplary structure of asemiconductor-nanocluster hybrid sensor is illustrated. Referring toFIG. 9, plate (a), the sensor may comprise a two-terminal sub-micronwide semiconductor backbone, functionalized with nanoclusters ofmetal-oxides and/or metals. For example, the sensor may include alightly-doped 0.8-0.25 μm wide semiconductor two-terminal structure on anon-conductive substrate (e.g. sapphire) formed using traditional deepUV photolithography and plasma etching. Functionalization is adiscontinuous layer of multicomponent nanoclusters (e.g., eachnanocluster comprising one or more photocatalytic metal-oxidenanoclusters (diameter 20 nm and smaller) and smaller metalnanoparticles (5 nm and smaller) deposited on top of it). Themulticomponent design may include more than one oxide and metal types inthe nanoclusters, and exhibits tailored adsorption properties by virtueof the multicomponent design. The functionalization layer is depositedusing reactive sputtering technique followed by thermal treatment—allstandard semiconductor microfabrication processes. The sensors work withlow-intensity light, such as from an LED. The emission wavelength isdetermined by the semiconductor and metal-oxide bandgaps. FIG. 9, plate(b) illustrates schematically an exemplary thin-film device including asemiconductor backbone functionalized with TiO₂ on a sapphire substrate.The smoothness of the substrate and film after thermal processing isshown in FIG. 9, plates (c) and (d).

Surface defects of metal-oxides are the active sites for adsorption ofvarious chemicals. However, at room-temperature the adsorbed oxygen andwater are very stable. This necessitates heating in traditionalmetal-oxides sensors. Most metal-oxides are well-know photocatalysts,with photoexcitation wavelengths in the range of ultraviolet to visible,corresponding to the material bandgap. A disclosed approach uses dynamicsurface-defects generation in the metal-oxide cluster throughillumination, which allows for efficient photodesorption of adsorbedwater and oxygen. This has at least two benefits: 1) low-power,room-temperature operation, which also increases the lifetime of thesensors, and 2) real-time dynamic range modulation by changing theintensity of light (for ppt level detection the intensity of the LED canbe increased as compared to ppm level detection).

The sensor architecture provides for the combination of a crystallinetop-down fabricated semiconductor backbone with a discontinuousnanocluster surface layer. In metal-oxide gas sensors, the resistancechanges due to diffusion and adsorption of gases along the grainboundaries. As the present architecture uses a discontinuous,nano-island like metal-oxide layer, the bottleneck of gas diffusionthrough grain boundaries, as in traditional metal-oxide sensors, is notpresent. This makes the disclosed sensors respond relatively fast ascompared to conventional sensors, and operable at room-temperature.Unlike traditional metal-oxide sensors, the disclosed design providesthat the current is carried by the high-quality, high mobilitysemiconductor backbone, which makes the sensor fast. Also, the absenceof conduction in the nanocluster layer makes the active layer inherentlystable as compared to traditional metal-oxide thin film sensors (e.g.,grain boundary motion, defect generation and propagation, and reductionof the metal-oxide layer is not possible due to the absence of a“closed-circuit”).

Due to the nanocluster layer of the disclosed sensors, designed with aspecific adsorption profile, they are extremely efficient in adsorbingtarget analytes. This enables the design of highly-selective sensors.Two component, three component, four component, or five or morecomponent cluster designs are possible for unprecedented selectivitytailoring.

Most semiconductors have depletion regions associated with them. Thesurface band bending, which is a consequence of the surface depletion,facilitates the diffusion of the photogenerated holes to the surface.This separation of carriers effectively suppresses their recombination.The degree of separation is determined by the surface potentialmodification by the clusters. Such separation of photocarriers increasestheir lifetimes, leading to higher photocurrent and thus sensitivitytowards such surface potential modifications. The processes that enablesensing of different adsorbed molecules with the disclosedmulticomponent nanocluster functionalization is shown schematically inFIG. 10.

Assuming typical values of the response/recovery times for 500 ppt ofNO₂, from the kinetic theory of gases the flux F of NO₂ arriving on asurface is given by the formula:

$\begin{matrix}{F = \frac{N_{A}P_{partial}}{\sqrt{2\pi\;{MRT}}}} & \left( {{Equation}\mspace{14mu} 7} \right)\end{matrix}$

where N_(A) is the Avagadros' number, M is the average molar weight ofthe molecule, P is the pressure, T is the temperature, and R is the gasconstant.

For 500 ppt concentration of NO₂ in air, three molecules of NO₂ areimpinging on a 20 nm diameter metal-oxide cluster per second. Now, theresidence time τ of an adsorbate at temperature T on a surface is givenby the relation τ=τ0 exp(ΔH_(ads)/RT), where ΔH_(ads) is the heat ofadsorption, and τ0 is correlated with surface atom vibration (roughly10⁻¹² s). Thus, at 298 K the residence time for NO₂ molecule on WO₃nanocluster is approximately 15 seconds (considering ΔH_(ads) for NO₂ onWO₃ to be 18 kcal/mol). Considering roughly 10²¹ cm⁻³ of defect densityfor typical metal oxides, results in roughly 300 adsorption sites on a20 nm diameter nanocluster. Assuming sticking coefficient of 1, by 110seconds the surface defects are saturated. Thus, response time may beestimated to be in the order to 100 seconds, and recovery time in theorder to 15-30 seconds. Although the design of the nanocluster isdescribed from pure thermodynamic standpoint, other surface kinetics(such as diffusion, desorption) may also be considered.

For fabricating the sensor backbone, un-doped (1×10¹⁶ cm⁻³) to lightlydoped (1×10¹⁷ cm⁻³) semiconductor epitaxial layer (1 μm thick) onsapphire/insulating/semi-insulating substrates may be utilized, as shownin FIG. 11. Lower doping is needed for the sensors to be photo enabled.The thickness of buffer layer controls the defects arising from latticeand thermal mismatch. Ideally suited layer structures require arelatively thin buffer layer (e.g., about 250 nm) to suppress theparasitic conduction in the buffer layer. Similar designs may also beprovided with other direct gap semiconductors, such as ZnO, InN, AlGaNand virtually any other direct gap semiconductor material.

The design of submicron semiconductor backbone including physical layoutand geometry is described with reference to FIG. 12. Both serial andparallel architectures for the semiconducting resistive backbone haveunique advantages and disadvantages as the chemiresistor backbone.Serial architecture has higher resistance which results in lower-poweroperation, whereas parallel architecture produces more robust devicesinsensitive to material quality variation in the individual sections.However, the calculation will show that the response R is the same forboth serial and parallel architecture:

$\begin{matrix}{R = \frac{R_{analyte} - R_{air}}{R_{air}}} & \left( {{Equation}\mspace{14mu} 8} \right)\end{matrix}$

wherein R_(analyte) and R_(air) are the resistances in presence ofanalyte and in air, respectively. However, the resolution of the sensor(i.e., smallest change in concentration it can measure as required forproposed large dynamic range sensors) is greater in a serialarchitecture.

The series sensor element provides for a meander shape, with integratedpassive sections as real-time calibration elements. An exemplary designis shown in FIG. 13, plate (a). The surface area-volume ratio for thisstructure is roughly 3.1. The sidewalls of the backbone may beintentionally angled, such as at 85° as shown in FIG. 13, plate (b).This ensures uniform coverage of the nanoclusters on the sidewalls ofthe structure, and also ensures uniform photoexcitation of thesemiconductor backbone. The device is biased by a standard three dcvoltage source (two AA batteries in series) and the sensor output is thevoltage measured between the pads +V_(sensor) and ground. The designprovides various benefits including: 1) high sensitivity and resolution;2) low-power consumption; 3) simplified interface circuit; and 4)ability for real-time base-line drift calibration and temperaturecompensation even in presence of analytes.

Using circuit analysis, it can be shown that Sensitivity S (as definedin FIG. 14) may be simplified considering R_(L)>>R as:

$\begin{matrix}{S = {\frac{R_{L} \times V_{dc}}{NR}\left\lbrack \frac{1}{\frac{R}{\Delta\; R} + 1} \right\rbrack}} & \left( {{Equation}\mspace{14mu} 9} \right)\end{matrix}$wherein R_(L) is the external low-noise precision load resistance (e.g.,see FIG. 13, plate (a)), N is the number of segments, R is theresistance without analyte of single segment, and ΔR is the resistancechange of the single segment in presence of the analyte, and V_(dc) isthe dc source voltage.

Thus for higher sensitivity, N should be small, and R_(L) and V_(dc)should be large. However, resolution of a sensor is the smallest changein concentration of the analyte it can measure (it is different fromlowest detection limit), and is often limited by the noise. Consideringonly thermal noise current in the total sensor, the output sensorvoltage noise can be expressed as:

$\begin{matrix}{{V_{sensor}({noise})} = {R_{L}\sqrt{\frac{4k_{B}T\;\Delta\; f}{NR}}}} & \left( {{Equation}\mspace{14mu} 10} \right)\end{matrix}$

wherein k_(B) is the Boltzmann Constant, T is the temperature, and Δf isthe bandwidth. Considering both Equations 9 and 10, the tradeoff betweenhigh sensitivity and resolution is clear. The effect of N (i.e., numberof segments) on the sensor performances such as sensitivity, detectionlimits, and resolution, may be investigated.

Referring again to FIG. 13, plate (a), the resistance of the activesensor area may be computed using the formula, neglecting the bends:

$\begin{matrix}{R \approx \frac{\rho \times \left( {4 \times L_{a}} \right)}{h \times {\left( {W_{u} + W_{b}} \right)/2}}} & \left( {{Equaiton}\mspace{14mu} 11} \right)\end{matrix}$

wherein ρ=1/(nqμ), ρ is the resistivity, n is the carrier concentration,and μ is the mobility (see also dimensions shown in FIG. 13, plate (b)).

For example, for the GaN backbone with dimensions shown in FIG. 13, theactive-area photoresistance under 365 nm excitation from LED is ≈60 kΩ,assuming a mobility of 300 cm²V⁻¹ s⁻¹ and electron concentration of1×10¹⁷ cm⁻³. The device is considered to be excited by low-intensity (10μW/cm²) 365 nm LED. The GaN absorption coefficient α=10⁵ cm⁻¹ for the365 nm photon is assumed. If the sensor is biased with 3 V dc and withan external 10 KΩ resistor, the power dissipation is approximately only40 μW. The sensor power dissipation when in offstate (LED turned off andthe sensor has only dark current) is even lower. The total powerrequirement for the sensor must also include the power required for LEDoperation. There are several low-power UV (365 nm) LEDs (FOX GROUP) thatcould be run by LED drivers. Power dissipation for the LED could be lowas 0.5 mW, if we drive the LED for very low intensity. Using a LEDdriver to control the intensity has an added benefit of the real-timedynamic range configuration.

The simplified chemiresistive architecture lends itself easily tointegration with interface devices as compared to more complex devicessuch as metal-oxide-semiconductor field-effect transistors (MOSFETs).The nano-watt operation amplifier (OP-Amp) TS1001 from TouchstoneSemiconductor is identified, which can provide a gain of 100 whenoperated in single-input voltage amplifier configuration. The Op-Ampoperated from a single AA battery dissipated about 1 μW.

In one implementation, a feature of the present design is the inclusionof the voltage probes (V_(cal)) for calibration of base line drift ofthe photoresistance of the total structure. As the area under thecalibration probes is encapsulated with thick SiO₂, the voltage drop(V_(cal)) for a fixed intensity of illumination through the entirestructure will enable compensation for drift in the baselinephotoresistance arising from persistence photoconductivity ortemperature-induced drift.

Another feature of the present design is the “tailored” adsorptionprofile through the multicomponent nanocluster design, as describedabove. The design provides for suppressing the competitive adsorption ofan interfering chemical on a surface with two different adsorptionprofiles, which is achieved using a primary and a secondary component.

In this regard, FIG. 16 illustrates an exemplary multicomponent designfor the target analyte of NO₂ and for the interfering chemical of CO₂.Adsorption profile for another target analyte or set of analytes alongwith a set of interfering chemicals may alternatively be providedutilizing a similar configuration. The primary metal-oxide component ischosen so that the heat of adsorption of NO₂ on its surface is largecompared to CO₂. The secondary component (e.g., the metal) is chosenwith the heat of adsorption for CO₂ larger than the metal-oxide. ThusNO₂ and CO₂ preferentially adsorb on the metal-oxide and the metal,respectively. When NO₂ is adsorbed on the metal-oxide, it interacts withthe photogenerated charge carriers, producing modulation of thesemiconductor backbone photocurrent, as explained above. However, whenCO₂ is adsorbed on the metal, due to the large concentration ofelectrons, there is minor change in the cluster potential. Considerationof other effects, such as catalytic decomposition on the metal,spill-over from the metal to metal-oxide, and change of metal-workfunction due to adsorption of gases, may also be appropriate.

Due to the highly dispersed nature of the metal phase, even if there isa change in the physical properties of the metals, it has only marginalimpact on the cluster properties. Although the general design principlesare described, the specific designs of the appropriate clusters may befine-tuned for optimal performance and selectivity. For example, Table Vbelow demonstrates possible cluster designs for NO₂ and benzene sensing.Considering the heat of adsorption of NO₂ on WO₃ and Pt, bigger WO₃clusters with much smaller and dispersed phase of Pt may be favorable.Although, adsorption energy for NO₂ is comparable on both WO₃ and Pt,due the higher surface area of metal-oxide clusters, most of NO₂ willadsorb on the WO₃, whereas CO₂ will mostly adsorb on the Pt. For BTEXsensing, the TiO₂/Fe is favorable.

TABLE V Heat of adsorption on different candidates for themulticomponent cluster design. Possible Cluster Designs for NO₂ sensing:NO₂ CO₂ Metal-Oxide/Metals (kcal/mol) (kcal/mol) MgO 9.0 3.5 TiO₂ 21.029 WO₃ 18.4 negligible Fe(111) 64.5 69 Pt(111) 19 40.5 Possible ClusterDesigns for Benzene sensing: Benzene CO₂ Metal-Oxide/Metals (kcal/mol)(kcal/mol) TiO₂ 15.2 29 Fe (111) 22 69

Note that the values in Table V are average adsorption energies at roomtemperature for low adsorbate coverage. The values are collected fromexperimental results (temperature programmed desorption and calorimetricstudies) and theoretical calculations (such as density function theory).The values shown are for common and stable oxide surfaces. Experimentalheat of adsorption values are dependent on various factors, includingthe morphology and crystal orientation of the surface.

Other design considerations for the nanoclusters include:

1) Bandgap of the oxide: as single wavelength excitation is used forboth photodesorption of surface oxygen and hydroxyl species, and forcreating photocarriers in the semiconductor (e.g. GaN), the bandgap ofthe oxide should be lower or equal to GaN bandgap (as shown in FIG. 15).Candidates are shown in Table VI below.

TABLE VI Bandgaps of Common Metal Oxides Metal-Oxides Bandgap (eV) MgO7.1 TiO ₂ 3.2 WO ₃ 2.8 Fe ₂ O ₃ 2.1 V ₂ O ₅ 2.3 NiO 3.6 Al₂O₃ 7.0Candidates are in bold and underlined, E_(g) < 3.4 eV.

2) Nature of surface defect types: surface defects (i.e. the activeadsorption sites) of metal-oxides are of three types: bronstead,lewis-acid/base sites, and redox sties. Organic compounds such asbenzene predominantly adsorb by dehydgrogenetion (i.e., removal of H+)requiring surface lewis sites. On the other hand, NO₂ predominantlyadsorbs as surface nitrate (NO₃ ⁻), requiring base sites. Mostmetal-oxide surfaces at room-temperature are hydroxylated, and thusphotoexcitation will increase the concentration of one type ofpredominant defects.

3) Redox potentials of the oxide: redox potentials of oxides indicatethe ability of photogenerated carriers to oxidize or reduce any adsorbedmolecule. Depending on whether molecules will be oxidized or reduced onthe surface, they interact with charge carriers differently in theclusters.

4) Stability of the adsorbates: Stability of the adsorbed species is animportant consideration, as it determines the recovery time, andultimately usability of the sensors. As can be seen for Fe, where thevery high adsorption energy might produce very stable NO adsorbedspecies on the surface, rendering the nanoclusters inactive afterexposure to high concentrations of NO₂.

5) Nature of the adsorbed species (molecular or dissociative): nature ofthe adsorbed species determines the photochemical reaction pathways andultimately the sensitivity. Additional multicomponent nanoclusterdesigns for NO₂ and BTEX sensing are shown in Table VII.

TABLE VII Possible designs of nanoclusters Metal-Oxides/Metal TargetAnalyte WO₃/Pt NO₂ TiO₂/Fe BTEX

The use of heterogeneous metal-oxide supported metal catalysts inindustrial production, abatement, and remediation for the past fewdecades has been extensive, and generated an exhaustive body ofliterature that may be readily utilized for nanocluster designsaccording to the present invention. Indeed, some of the systems arewell-understood, so that a desired selectivity outcome may be readilypredicted. The well-known strong metal/support interactions (SMSI)effects in heterocatalysts are different, as the metals are not reducedon the oxides in the disclosed devices.

Computing the size and coverage of the clusters is an importantconsideration, given the size and coverage of the NCs ultimatelydetermines the overall sensitivity of the device. Thus, determination ofthe most effective size and coverage of the clusters is desirable. It isknown that the surface area and relative particle size has a significanteffect on the catalytic properties of metals and metal oxides. However,due to the presence of metals on the metal-oxide clusters, there will besignificant depletion of the metal-oxide clusters. Thus, overly smallmetal-oxide clusters would be substantially depleted and hampereffectiveness, whereas overly large clusters would also result in lowersensitivity. Consideration of the nature of the depletion regions formedby such nano-sized metal clusters on a semiconductor is thereforeprudent.

The classical Schottky model depletion theory cannot predict accuratelythe zero-bias depletion width produced by metallic nanoclusters on asemiconductor. According to Zhdanov's model, the depletion depthassociated with such metal nanoclusters on a semiconductor can beestimated by the following relationship:

$\begin{matrix}{w_{d} = \left( \frac{3r_{c}V_{bi}}{2\pi\; q^{2}N_{d}} \right)^{1/3}} & \left( {{Equation}\mspace{14mu} 12} \right)\end{matrix}$

wherein w_(d) is the depletion width, r_(c) is the radius of thenanocluster, V_(b) _(i) is the built-in voltage for the junction, q isthe elementary charge, and N_(d) is the dopant concentration in thesemiconductor.

The plot in FIG. 17 demonstrates the depletion width of TiO₂ clustersdue to Pt particles. It is clear that 4 nm of Pt clusters on 20 nmdiameter TiO₂ clusters would produce depletion of about 5 nm in theTiO2.

Coverage of the metal-oxide nanocluster functionalization is determinedby the limit of formation of continues metal-oxide film. The coverage isdependent on various parameters such as metal-oxide wetting of thesemiconductor, morphology of phases formed after thermal treatment,etc., and may be verified by SEM imaging. The metal coverage should besparse to ensure only partial depletion of the clusters.

With regard to fabrication, techniques such as wet chemical etching maynot be suitable for etching nanoscale, high aspect-ratio nanostructuresdue to undercutting of the mask and sloped sidewalls. Hence, thedevelopment of a dry etching process with relatively less low damage andprecise-depth control capability is preferred for the fabrication ofnanostructures. Such etching of semiconductor nanostructures isdescribed in further detail in Example 4 below.

Referring to FIG. 18, the components for an exemplar interface circuitis illustrated. The LED intensity may be controlled by themicrocontroller (MAXQ3213, with a LED driver). By relatively simpledesign change of a selected multicomponent cluster, differentapplications are readily provided. In addition, using wide bandgapmaterial as a backbone enables the sensor to work at elevatedtemperatures, and in presence of radiation and other harsh environmentalconditions.

As shown in Table VIII below, the possible designs of themulti-component nanoclusters are virtually unlimited, resulting in theability to provide sensors for numerous applications.

TABLE VIII Exemplary Designs of Multicomponent Nanoclusters NanoclusterComponents: Semiconductor Metal Oxide: Metal: GaN Titanium OxideTitanium InN Tin Oxide Nickel AlGaN Iron Oxide Chromium Magnesium OxideCobalt Vanadium Oxide Ruthenium Nickel Oxide Rhodium ZnO Zirconium OxideGold InAs Aluminum Oxide Silver Copper Oxide Platinum Zinc OxidePalladium Strontium Oxide Vandium

Thus, in accordance with the disclosed methodologies, sensor devicessuitable for a wide range of applications are achieved. Further, theparticular architecture of the sensor devices may be readily tailoredfor the desired application and associated conditions, as well as one ormultiple active sensor elements configured for sensing particle targets.For example, an exemplary sensor device includes eight individuallyaddressable active sensor elements, as shown in FIG. 53, which can eachdetect a different target analyte (e.g., various gases). The sensordevice may include an on-chip calibration element for automatic driftcompensation. The sensor device may also include an on-chipmicro-heater, as shown in FIG. 54, for stabilizing temperature and/orhumidity. The temperature profiles of a 50 μm microheater made from aTi/Ni metal stack MH recorded at 5 V bias voltage and 10 V bias voltageare shown in FIG. 55, plates (a) and (b), respectively.

Thus, the disclosed sensor devices may comprise various active sensorelements and passive elements for formation of on-chip circuits.Multiple active elements may be provided with a combination of differentfunctionalization to detect multiple gases in a single chip. The chipmay include precise passive elements (elements which have the samesemiconductor backbone but passivated from the environment), forcalibration on the same chip, which has the same temperature coefficientfor current as the active sensor element. Thus, any change due to thetemperature or aging can be a calibrated out using the on-chipcalibration element(s). Using such on-chip components (e.g., see FIG.53), bridge circuits may be provided directly on the chip, allowing forsensor devices with high resolution.

Although the sensor devices may comprise a micro-heater element as notedabove, such element is not required. The disclosed sensor devices do notneed to be heated for sensing, and are capable of sensing a host ofgases at room temperature. Total power consumption is extremely low(e.g., an exemplary 8 active sensor element device provided for a totalpower consumption about 10 microwatts. Further, the disclosed sensordevices are stable and recoverable even in the presence of corrosivegases (e.g, HCN, CL₂, HCl, etc), and capable of withstanding very highgas concentrations. The sensor devices are also capable of operating inoxygen rich or relatively lean conditions.

In accordance with disclosed embodiments, the active sensor(s) elementsare designed by first selecting a nanoclusters and/or a layer of a basephotocatalytic metal oxide (e.g., TiO₂, V₂O₅, Cr₂O₃, Fe2O3, CoO, NiO,CuO, ZnO, ZrO₂, WO₃, MoO₃, SnO₂). Nanoclusters of a catalytic metal(e.g., Ti, V, Cr, Fe, Co, Ni, Cu, Al, Zr, Nb, Mo, Ru, Rh, Pd, Ag, Hf,Ta, W, Re, Ir, Pt, Au) are then applied on top of the basephotocatalytic metal oxide nanoclusters. Alternatively in otherembodiments, nanoclusters of a second photocatalytic metal oxidedifferent than the base metal oxide are applied on top of the base metaloxide, providing for dual metal oxide functionalizations. Thus, thesensor element comprises a base layer or nanoclusters of a firstmetal-oxide, and nanoclusters of a second metal oxide or metal. Theselection of the particular metal oxide and metal provides for thedesired selectively.

For example, the dynamic response of functionalized GaN NW with selectedmetal oxide for selectively sensing hydrogen sulfide (H₂S) in dry air isshown in FIG. 56. The response of the functionalized GaN NW for sensingNO₂ in dry air is shown in FIG. 57. The response of the functionalizedGaN NW for sensing SO₂ in dry air is shown in FIG. 58. The response ofthe functionalized GaN NW for sensing CO₂ in dry air using the metaloxide sensor devices of the present invention is shown in FIG. 59. Thus,a wide range of target gases, from reducing to oxidizing to inert gases,is achieved.

A summary of operational and performance specifications of sensingdevices in accordance with disclosed embodiments is set forth in TableIX below:

Response (%) = Analyte Range of Detection (R_(gas) − R_(air)/R_(air))Ammonia 1-100 ppm 15 Chlorine 0.5-10 ppm  212 Hydrogen chloride 1-100ppm 74 Hydrogen cyanide 1-100 ppm 10 Hydrogen sulphide 10-1000 ppm  20Hydrogen 0.5-10%  500 Oxygen 10-30% 40 Carbon dioxide  01.-1% 2 Carbonmonoxide 10-300 ppm  15 Nitrogen dioxide 100-500 ppm  2 Nitric oxide5-1000 ppm  2.6 Methane 50-5000 ppm  9

The disclosed devices are suitable for environmental monitoring, hazmat,large-scale industrial monitoring and control, explosive threatdetection, and other markets where rapid detection of gases andchemicals in air is desired. Compared to conventional sensors, thedisclosed sensors of the present invention are extremely small (e.g., 4mm×4 mm, or 2.5 mm×2.5 mm, or smaller) and inexpensive, exhibit lowpower consumption (e.g., less than 100 microwatts, and in someembodiments less than about 10 microwatts), but capable of sensing alarge dynamic range (e.g., 100 parts per billion to >2%), detect avariety of chemicals under various conditions with no cross-sensitivity(thus minimizing false positives), and exhibit a long operating life. Inaddition, the disclosed sensors of the present invention may bemanufactured using the same manufacturing methodologies utilized forproducing conventional integrated circuits. An exemplary sensor moduleis shown in FIG. 60, which has dimensions of about 8 cm×6 cm×1 cm, aweight of 0.4 pounds, power consumption requirements for continuousoperation of about 0.2 watts, eight active sensor elements or channelsfor simultaneous measurement of eight different target gases, andincluding a built-in air sampling element with microblower.

The disclosed sensor devices may be installed in residential andcommercial buildings for on-demand ventilation control, resulting in adecrease in energy consumption. The sensors can detect the presence ofharmful VOCs (Benzene, Xylene, and formaldehyde), which are oftenemitted by building materials, paints, and furniture, and are alsoassociated with human metabolism. After detecting an increase in thelevels of targeted harmful chemicals, the ventilation system may beadjusted for safety, comfort and health of the occupants. Alternativelyor in addition, the sensors could monitor CO levels and gas leaks inbuildings for safety. Thus, the disclosed sensor technology may bereadily implemented in indoor monitoring systems, thereby generatinglarge cost savings in terms of energy efficiency, health of theoccupants, and low-maintenance costs.

In case of accidental release of chemicals, the disclosed sensors aresuitable for use by first-responders to detect the presence of chemicalsand associated hazards. Thus, the challenges of a disaster may bemanaged more safely and efficiently. The disclosed hybrid sensortechnology may be implemented in ultra-small, handheld units, whichidentify multiple hazardous materials with low power consumption. Suchdevices would be ideal for first responders.

The disclosed sensors are also suitable for industrial monitoringapplications. For example, the sensors may be used for monitoringdifferent gases for process control in industrial facilities such as oilrefineries, manufacturing plants, etc. They may be installed at variouspoints throughout an industrial facility for point detection for leaksof toxic chemicals. The may also be implemented in personal monitoringdevices for recording personal exposure levels for compliance purposeswith state and federal maximum exposure level regulations. The disclosedtechnology therefore promises unlimited control over the sensor design,thus having the ability to produce sensors for various differentindustries and processes.

Implementations of the disclosed technology for law enforcement andsafety applications are also provided. For example, the disclosedsensors may be utilized in breath analyzers for law-enforcement andindividual use. The hybrid sensors may also be integrated into hand-helddevices (e.g., cell phones) as plug-in modules to existing devices. Forexample, the disclosed sensor may be integrated into a hand-held deviceto enable a user to check his or her blood alcohol level.

Implementations of the disclosed sensor technology are also suitable fordefense and security applications. The sensors may be used for safetymonitoring in public places such as subway/rail stations, airports,public buildings, and in transit systems. For example, the sensors maybe utilized to monitor and detect deliberate release of harmfulchemicals and explosives, thus protecting civilians from attacks. Theymay also be integrated into equipment carried or worn by soldiers fordetection of harmful chemicals, explosives, or other terrorist elements.

Having generally described the invention, the same will be furtherunderstood through reference to the following additional examples, whichare provided by way of illustration and are not intended to be limitingof the present invention unless specified.

EXAMPLES Example 1

Nanowire-nanocluster hybrid chemical sensors were realized byfunctionalizing gallium nitride (GaN) nanowires (NWs) with titaniumdioxide (TiO₂) nanoclusters for selectively sensing benzene and otherrelated aromatic compounds.

Materials and Methods

C-axis, n-type, Si-doped GaN grown by catalyst-free molecular beamepitaxy on Si (111) substrates were utilized. For details of NW growth,see Bertness K A et al. (2008) “Mechanism for spontaneous growth of GaNnanowires with molecular beam epitaxy,” J. Crystal Growth310(13):3154-3158). An exemplary process of sensor fabrication is shownin FIG. 19. Post-growth device fabrication was done bydielectrophoretically aligning the nanowires on 9 mm×9 mm sapphiresubstrates (see Motayed A et al. (2006) “Realization of reliable GaNnanowire transistors utilizing dielectrophoretic alignment technique,”J. Appl. Phy. 100:114310). The device substrates had 12 nm thick Tialignment electrodes of semi-circular geometry with gaps between themranging from 4 μm to 8 μm. After the alignment of the nanowires, thesamples were dried at 75° C. for 10 min on a hot plate for evaporationof the residual solvent. This was followed by a plasma enhanced chemicalvapor deposition (PECVD) of 50 nm of SiO₂, at a deposition temperatureof 300° C. This passivation layer was deposited to ensure higher yieldfor the fabrication process.

After the oxide deposition, photolithography was performed to defineopenings for the top contact. The oxide in the openings was etched usingreactive ion etching (RIE) with CF₄/CHF₃/O₂ (50 sccm/25 sccm/5 sccm) gaschemistry. The top contact metallization was deposited in anelectron-beam evaporator with base pressure of 10⁻⁵ Pa. The depositionsequence was Ti (70 nm)/Al (70 nm)/Ti (40 nm)/Au (40 nm). The oxidelayer over the nanowires between the end contacts was then etched inbuffered HF etching solution for 15 seconds. A negative resist was usedto protect the end metal contacts from the etching solution.

The TiO₂ nanoclusters were deposited on the exposed GaN NWs using RFmagnetron sputtering. The deposition was done at 325° C. with 50 sccm ofAr flow, and 300 W RF power. The deposition rate was about 0.2 Å/s.Thermal annealing of the complete sensor devices (GaN NW with TiO₂nanoclusters) was done at 650° C. to 700° C. for 30 seconds in a rapidthermal processing system with 6 slpm (standard liter per min) flow ofultrahigh purity Ar. A relatively slow ramp rate of 100° C. per min waschosen to reduce the stress in the metal-nanowire contact area duringheating. The anneal step was optimized to facilitate Ohmic contactformation to the GaN NWs and also to induce crystallization of the TiO₂clusters. Additional lithography was performed to form thick metal bondpads with Ti (40 nm) and Au (160 nm).

The crystallinity and phase analysis of the sputtered TiO₂ films wereassessed by X-ray diffraction (XRD). The XRD scans were collected on aBruker-AXS D8 scanning X-ray micro-diffractometer equipped with ageneral area detector diffraction system (GADDS) using Cu-Kα radiation.The two-dimensional 2Θ-χ patterns were collected in the 2Θ=23° to 51°range followed by integration into conventional Ω-Θ scans. Themicrostructure and morphology of the sputtered TiO₂ films used forfabrication of sensors were characterized by high-resolution analyticaltransmission and scanning transmission electron microscopy (HRTEM/STEM)and cold field-emission scanning electron microscopy (FESEM). GaNnanowires with sputtered TiO₂ were deposited onto a lacey carbon filmssupported by Cu-mesh grids and analyzed in a 300 kV TEM/STEM microscope.The instrument was equipped with an X-ray energy dispersive spectrometer(XEDS) and an electron energy-loss spectrometer (EELS) as well asbright-field (BF) and annular dark-field (ADF) STEM detectors to performspot and line profile analyses.

The device substrates, i.e., the sensor chips, were wire-bonded on a 24pin ceramic package for the gas sensing measurements. The devicecharacterization and the time dependent sensing measurements were doneusing an Agilent B1500A semiconductor parameter analyzer. Each sensorchip was placed in a custom-designed stainless steel test chamber ofvolume 0.73 cm³ with separate gas inlet and outlet. The test chamber hada quartz window on top for UV excitation provided by a 25 W deuteriumbulb (DH-2000-BAL, Ocean Optics) connected to a 600 μm diameter opticalfiber cable with a collimating lens at the end for uniform illuminationover the sample surface. The operating wavelength range of the bulb was215 to 400 nm. The intensity at 365 nm measured using an optical powermeter was 375 nW cm⁻². For all the sensing experiments regular breathingair (<9 ppm of water) was used as the carrier gas. A wide range ofconcentrations from 1% to as low as 50 parts per billion (ppb) ofvarious organic compounds were achieved with a specific arrangement ofbubbler and mass flow controllers (MFCs). During the sensormeasurements, the net flow (air+VOC mix) into the test chamber was setto a constant value of 20 sccm. After the sensor devices were exposed tothe organic compounds, they were allowed to regain their baselinecurrent with the air-chemical mixture turned-off, without purging orevacuating the test-chamber.

Results

FIG. 20 shows GaN nanowires with three different nominal thicknesses ofTiO₂ coatings sputtered on them: 2 nm (FIG. 20, plate (a)); 5 nm (FIG.20, plate (b)); and 8 nm (FIG. 20, plate (c)). Rather sparse,well-defined clusters can be seen for both the 5 nm and 8 nmarea-averaged sputtered coatings of TiO₂. The average size of theselarge clusters was about 15 nm. For the 8 nm sputtered coating, thecoverage of the TiO₂ clusters is much denser. However, TEM studiesrevealed the presence of clusters with much smaller diameter (less thanabout 4 nm) on the nanowire surface.

Detection of XRD signal from the TiO₂ decorated GaN NWs was difficultdue to the minuscule size and total volume of TiO₂ nanoclusters. Wetherefore prepared a 150 nm thick TiO₂ film by sputtering it onto a SiO₂coated Si substrate at 300° C. followed by anneal at 650° C. for 45 s inargon. The processing conditions produced an identical morphology as inthe TiO₂ decorated NW case. Referring to FIG. 21, we identified from theXRD that TiO₂ is in the single-phase anatase form. As-deposited TiO₂films were found to be amorphous.

The XRD results agree with the TEM analysis on TiO₂ decorated GaN NWs,which revealed that upon annealing at 700° C. for 30 s, the TiO₂ islandsbecame partially crystalline, as shown in FIG. 22. Three most commonphases of TiO₂ are anatase, rutile, and brookite. Thermodynamiccalculations predict that rutile is the most stable TiO₂ phase in thebulk state at all temperatures and atmospheric pressure (see Norotsky Aet al. (1967) “Enthalpy of Transformation of a High-Pressure Polymorphof Titanium Dioxide to the Rutile Modification,” Science 158:338;Jamieson J C and Olinger B (1969) “Pressure-temperature studies ofanatase, brookite, rutile, and TiO ₂ (II); A discussion,” Am. Min.54:1477-1480). However, comparative experiments with particle sizeshowed that the phase stability might reverse with decreasing particlesize, possibly due to the influence of surface free energy and surfacestress (Zhang H Z and Banfield J. F (2000) “Understanding polymorphicphase transformation behavior during growth of nanocrystallineaggregates: insights from TiO ₂,” J. Phys. Chem. B 104:3481-3487).Anatase is the most stable phase when the particle size is less thanabout 11 nm, whereas rutile is most stable at sizes greater than about35 nm. Although both rutile and anatase TiO₂ are commonly used asphotocatalyst, anatase form shows greater photocatalytic activity formost reactions (Linsbigler A L et al. (1995) “Photocatalysis on TiO ₂Surfaces: Principles, Mechanisms, and Selected Results,” Chem. Rev.95:735-7; Tanaka K et al. (1991) “Effect of crystallinity of TiO2 on itsphotocatalytic action,” Chem. Phys. Lett. 187:73-76). This is oneconsideration for sputtering nominally 8 nm of TiO₂ for the sensorfabrication.

Although we have sputtered 8 nm of TiO2 for fabricating the hybridsensors, for the TEM studies 20 nm of TiO₂ coating was utilized. Thethick GaN nanowires prevented acquisition of any TEM diffraction fromthinner TiO₂ coatings. The TEM results presented for 20 nm thick TiO₂was representative of the clusters formed for 8 nm deposited TiO₂ inactual sensors. Typical morphologies of a 20 nm thick TiO₂ filmsputtered on n-GaN nanowires and annealed at 700° C. for 30 seconds areillustrated by TEM data in FIG. 22. The TEM image in FIG. 22, plate (a)shows 2 nm to 10 nm diameter individual TiO₂ particles non-uniformlydistributed on the surface of a GaN nanowire. Some of the particles areidentified by circles. Crystallinity of some nanoparticles observed isshown in the HRTEM image in FIG. 22, plate (b) with nanocrystallites onthe edge of a GaN nanowire with the sputtered TiO₂. The FFT pattern fromthe boxed area is seen in exploded view in the upper left inset image,showing 0.35 nm lattice fringes which are consistent with a (101)reflecting plane of anatase but not available in hexagonal wurtzite-typeGaN crystals.

Referring to FIG. 23, plate (a), a BF-STEM image shows 5 to 10 nm TiO₂nanoparticles barely visible against the GaN nanowire. An ADF-STEM imageof a TiO₂ island on a GaN nanowire is shown in FIG. 23, plate (b). Thepresence of TiO₂ was confirmed by analysis of selected areas as well asof individual particles using XEDS and EELS and nanoprobe capabilities.Referring to FIG. 23, plate (c), the X-ray spectrum of an individual 5nm TiO₂ particle (identified by the marked circle “A” in FIG. 23, plate(a)) exhibits the TiKα peak at 4.51 keV and the weak Okα peak at 0.523keV. The NKα peak at 0.39 keV and gallium lines (the GaL series at 1.0keV to 1.2 keV) and the CKα peak at 0.28 keV are also identified. EELspectrum acquired at Position “1” marked in FIG. 23, plate (b) (the tipof a TiO₂-containing aggregate) exhibits the TiL edge at 456 eV and theOK edge at 532 eV and also the CK edge at 284 eV. A reference spectrumrecorded at Position 2 marked in FIG. 23, plate (b) (an edge of the GaNnanowire) reveals traces of titanium and oxygen with the NK edge at 401eV and the GaL edge at 1115 eV, respectively.

FIG. 24 shows the current-voltage (I-V) characteristics of a GaN NWtwo-terminal device at different stages of processing. The I-V curves ofthe as-deposited devices were non-linear and asymmetric. The currentdecreased when the SiO₂ layer over the NW was etched. However, thecurrent increased with the deposition of TiO₂ nanoclusters. Oxygenadsorption on the bare GaN nanowire surface can introduce surface states(Zywietz et al. (1999) “The adsorption of oxygen at GaN surfaces,” Appl.Phys. Lett. 74:1695), resulting in the decrease of the nanowireconductivity. The devices annealed at 700° C. for 30 seconds showedsignificant changes in their I-V characteristics with a majority of thedevices exhibiting linear I-V curves. This is consistent with the factthat low resistance ohmic contacts to the nitrides require annealing at700° C. to 800° C. (see Motayed A et al. (2003) “Electrical, thermal,and microstructural characteristics of Ti/Al/Ti/Au multilayer ohmiccontacts to n-type GaN,” J. Appl. Phys. 93(2):1087-1094).

FIG. 25 shows the photoconductance of a bare GaN NW device and the TiO₂coated GaN NW device. The NW devices with TiO₂ nanoclusters showedalmost two orders of magnitude increase in the current when exposed toUV light as compared to the similar diameter bare NW devices. Increaseof photoconductance due to surface functionalization has been observedin ZnO nanobelts coated with different polymers (Lao C S et al. (2007)“Giant Enhancement in UV Response of ZnO Nanobelts by PolymerSurface-Functionalization,” J. Am. Chem. Soc. 129:12096-12097). Thisenhancement of photoconductance is often attributed to the separation ofphotogenerated charge carriers by a surface depletion field, therebyincreasing the lifetime of the photogenerated carriers. After the lightis turned off, the photo current decays rapidly, but not to the darkcurrent value, which is likely due to the persistent photoconductivityof the NWs (see Sanford N A et al. (2010) “Steady-state and transientphotoconductivity in c-axis GaN nanowires grown bynitrogen-plasma-assisted molecular beam epitaxy,” J. Appl. Phy.107:034318).

The current through the bare GaN NW devices did not change when exposedto different VOCs mixed in air, even for concentrations as high as fewpercents. On the other hand, the TiO₂ coated hybrid devices respondedeven to the pulses of 20 sccm airflow. This is expected, consideringthat the conduction in most metal-oxides is affected by the presence ofoxygen. The response of the TiO₂ nanocluster-GaN nanowire hybrid sensorto 1000 ppm of toluene in air is illustrated in FIG. 26. Exposure to theVOC in the dark had no effect on the hybrid device. However, in presenceof UV excitation, when 1000 ppm of toluene (mixed in air) was introducedinto the gas chamber, the sensor photocurrent decreased dramatically toapproximately ⅔ of its base value. After 100 seconds of gas exposure,the gas flow is turned off and the sensor is allowed to recover at roomtemperature without any additional purging. The repeatability of thesensing action of these hybrid sensors is evident from FIG. 26.

Interestingly, the hybrid sensors did not respond when exposed tomethanol, ethanol, isopropanol, chloroform, acetone, and 1,3-hexadiene,even for concentrations as high as several percent. Also, thephotocurrent for these sensors increased with respect to air whenexposed to toluene vapors, whereas for every other aromatic compound,the photocurrent decreased relative to air, as shown in FIG. 27, plate(a). More than twenty sensor devices were tested, with all exhibitingthe same trend. In addition, the use of toluene from different sourcesresulting in the same sensor behavior. FIG. 27, plate (b) shows theresponse of a different device for 200 ppb concentrations of the samechemicals. It is clear that even for toluene concentration as low as 200ppb, the relative change in photocurrent is the reverse of that observedwith other chemicals. If the photocurrent in the presence of air forthese sensors is used as their baseline calibration, then we candistinctly identify toluene from other aromatic compounds present in airusing our hybrid devices. The response time is defined as the time takenby the sensor current to reach 90% of the response (I_(f)−I₀) whenexposed to the analyte. The I_(f) is the steady sensor current level inthe presence of the analyte, and I₀ is the current level without theanalyte, which in our case is in the presence of air. The recovery timeis the time required for the sensor current to recover to 30% of theresponse (I_(f)−I₀) after the gas flow is turned off (Garzella C et al.(2000) “TiO ₂ thin films by a novel sol-gel processing for gas sensorapplications,” Sens. and Actuators B: Chemical 68:189-196). The responseand recovery times for ppm levels of BTEX concentrations were ≈60seconds and ≈75 seconds, respectively. The response and recovery timesfor ppb levels of concentrations were ≈180 seconds and ≈150 seconds,respectively. In contrast, conventional nanowire/nanotube sensorsreported in the literature as working at room-temperatures had muchlonger response times in minutes (Leghrib R et al. (2010) “Gas sensorsbased on multiwall carbon nanotubes decorated with tin oxidenanoclusters,” Sens. and Actuators B: Chemical 145:411-416; Balazsi C etal. (2008) “Novel hexagonal WO3 nanopowder with metal decorated carbonnanotubes as NO2 gas sensor,” Sensors and Actuators B: Chemical133:151-155; Kuang Q et al. (2008) “Enhancing the photon-and gas-sensingproperties of a single SnO2 nanowire based nanodevice by nanoparticlesurface functionalization,” J. Phys. Chem. C 112:11539-11544; Lim W etal. (2008) “Room temperature hydrogen detection using Pd-coated GaNnanowires,” Appl. Phys. Lett. 93:072109). Fast response and recoverytimes indicate fast adsorption and desorption, which is attributed tothe enhanced reactivity of the nanoscale TiO₂ clusters.

The responses of two hybrid devices to different concentrations oftoluene in air are shown in FIG. 28. FIG. 28, plate (a) shows the changeof photocurrent of a 234 nm diameter device when exposed to tolueneconcentrations from 10000 ppm down to 100 ppm. FIG. 28, plate (b) showsthe photocurrent of a sensor device with 170 nm diameter wire fortoluene concentrations from 1 ppm to 50 ppb.

Sensitivity is defined as (R_(gas)−R_(air))/R_(air), where R_(gas),R_(air) are the resistances of the sensor in the presence of thechemical-air mixture and in the presence of air, respectively. Thesensitivity plots of a hybrid device for different VOCs tested are shownin FIG. 29. The sensitivity plot emphasizes the ability of these hybridsensors to reliably detect BTEX (benzene, toluene, ethylbenzene,chlorobenzene, and xylene), which are common indoor and outdoorpollutants with wide detection range (50 ppb to 1%).

Example 2

The sensing behavior of three NWNC based hybrid sensors was compared: 1)GaN NW coated with TiO₂ NCs (hereafter referred to as GaN/TiO₂ NWNChybrids); 2) GaN NW coated with TiO₂ and Pt multicomponent NCs (i.e.,GaN/(TiO₂—Pt) NWNC hybrids); and 3) GaN NW coated with Pt NCs (i.e.,GaN/Pt NWNC hybrids). It was found that sensors with TiO₂—Ptmulticomponent NCs on GaN NW were only sensitive to methanol, ethanol,and hydrogen. Higher carbon-containing alcohols (such as n-propanol,iso-propanol, n-butanol) did not produce any sensor response. Thesesensors had the highest sensitivity towards hydrogen. Prior to the Ptdeposition, the GaN/TiO₂ NWNC hybrids did not exhibit any response toalcohols, however they detected benzene and related aromatic compoundssuch as toluene, ethylbenzene, xylene, and chlorobenzene mixed with air.The GaN/Pt hybrids only showed sensitivity to hydrogen and not tomethanol or ethanol. The sensitivity of GaN/Pt hybrids towards hydrogenwas lower compared to the GaN/(TiO₂—Pt) hybrids.

Materials and Methods

GaN NWs utilized for this study were c-axis, n-type (Si-doped), grown bycatalyst-free molecular beam epitaxy as described by Bertness K A et al.(2008), supra, J. Crystal Growth 310(13):3154-3158. Post-growth devicefabrication was done by dielectrophoretically aligning the nanowires on9 mm×9 mm sapphire substrates. The details of the device fabrication areset forth in Example 1. After fabrication of two-terminal GaN NWdevices, the TiO₂ NCs were deposited on the GaN NW surface using RFmagnetron sputtering. The deposition was done at 325° C. with 50standard cubic centimeters per minute (sccm) of Ar flow, and 300 W RFpower. The nominal deposition rate was about 0.24 Å/s. Thermal annealingof the complete sensor devices (GaN NW with TiO₂ nanoclusters) was doneat 700° C. for 30 seconds in a rapid thermal processing system. ForTiO₂—Pt composite NCs, the Pt was sputtered using DC sputtering afterannealing of the TiO₂ clusters on GaN NW. The Pt sputtering was donewith an Ar flow of 35 sccm, at a pressure of 1.3 Pa and power of 40 Wfor 10 seconds. For the Pt/GaN devices Pt was sputtered on bare GaN NWsafter annealing the ohmic contacts at 700° C. for 30 seconds. Additionallithography was performed to form thick metal bond pads with Ti (40 nm)and Au (200 nm). The device substrates, i.e., the sensor chips, werewire-bonded on a 24 pin ceramic package for the gas sensingmeasurements.

The microstructure and morphology of the sputtered TiO₂ films used forthe fabrication of the sensors were characterized by high-resolutiontransmission and scanning transmission electron microscopy (HRTEM/STEM),selected-area electron diffraction (SAED), and field-emission scanningelectron microscopy (FESEM). For the TEM characterization, the GaN NWswere dispersed on 10 nm thick carbon films supported by Mo-mesh grids,followed by the deposition of TiO₂ NCs and annealing, and subsequent Ptdeposition. The samples were analyzed in a FEI Titan 80-300 TEM/STEMmicroscope operating at 300 kV accelerating voltage and equipped withS-TWIN objective lenses, which provided 0.13 nm (STEM) and 0.19 nm (TEM)resolution by points. The instrument also had a Gatan CCD imageacquisition camera, bright-field (BF), ADF and high-angle annulardark-field (HAADF) STEM detectors to perform spot, line profile, andareal compositional analyses using an EDAX 300 kV high-performance Si/LiX-ray energy dispersive spectrometer (XEDS).

The as-fabricated sensors were placed in a custom designed gas chamberfor gas exposure measurements. The device characterization and the timedependent sensing measurements were done using an Agilent B1500Asemiconductor parameter analyzer. The gas sensing experiments have beenperformed by measuring the electrical conductance of the devices uponexposure to controlled flow of air/chemical mixture in presence of UVexcitation (25 W deuterium bulb operating in the 215 nm to 400 nmrange). For all the sensing experiments with chemicals, breathing air(<9 μmol/mol of water) was used as the carrier gas. For the hydrogensensing we used high-purity nitrogen as the carrier gas. After thesensor devices were exposed to the organic compounds and hydrogen, theywere allowed to regain their baseline current with the air-chemicalmixture turned-off, without purging or evacuating the test-chamber.

Results

Morphological and Structural Characterization of NWNC Hybrids

It was challenging to measure the sizes and shapes of small TiO₂ and Ptparticles on the surfaces GaN NWs from greyscale TEM images due to: a)270 nm to 300 nm thickness of the NWs used in the devices and variationsof thickness and curvature across the structure; b) diffraction contrastinduced particularly by bending of the wires—even similar particlescould appear as having different intensities, while local thicknessvariations of the carbon support film could result in variable contrastaffecting the mean intensity values of the particles; c) overwhelmingdomination of electron diffraction in SAED from the GaN NW over thediffraction from TiO₂ and Pt nanoparticles. To overcome these problems,TEM imaging was conducted under minimal beam intensity conditions closeto the Scherzer defocus at highest available accelerating voltage of 300kV using both stationary beam (bright-field TEM/SAED, phase-contrasthigh-resolution TEM) and scanning beam (STEM/XEDS) modes. Areas foranalyses were selected near the wire's edges and on the amorphous carbonsupport film in the vicinity of the NWs.

FIG. 30 shows HRTEM micrographs of a GaN NW on a thin amorphous carbonsupport films with TiO₂ coating, before and after the Pt deposition. Thedeposited TiO₂ layer formed an island-like film, where 10 nm to 50 nmpartially aggregated particles (see FIG. 30, plate (a)) were ofteninterconnected into extended two-dimensional networks. This wasconsistent with SAED and compositional analyses of deposited TiO₂ filmsindicating a mixture of polycrystalline anatase and rutile and of thesame mixture plus fcc Pt nanoparticles (FIG. 30, plate (b)),respectively. Pt crystalline particles with 1 to 5 nm size were randomlydistributed on the surfaces of TiO₂ islands and sometimes were partiallycoalesced forming elongated aggregates. In the latter case, significantthickness of the GaN NWs made it difficult to visualize TiO₂ depositsdue to the limited contrast difference between TiO₂ and GaN and presenceof multiple heavy Pt particles. In spite of these limitations, detailedHRTEM and HR-STEM observations revealed 0.35 nm (101) hcp latticefringes belonging to anatase (see FIG. 30, plate (b), upper left inset)and 0.23 nm to 0.25 nm (111) and 0.20 nm to 0.22 nm (200) fcc latticefringes belonging to Pt nanocrystallites, respectively, as well asamorphous-like Pt clusters with diameters around 1 nm or less (see FIG.31, plates (a) and (b)).

In the FIG. 31, HAADF-STEM image shows 1 nm to 5 nm diameter bright Ptnanoparticles and barely visible TiO₂ islands (medium grey) randomlydistributed near the edge of the nanowire. The presence of both TiO₂ andPt nanocrystallites was confirmed by the analysis of selected areasusing XEDS nanoprobe capabilities.

Current-Voltage (I-V) Characteristics of NWNC Hybrids in Dark

FIG. 32 shows the I-V characteristics of the GaN/(TiO₂—Pt) and GaN/Pthybrid sensor devices at different stages of processing. A plan-view SEMimage of an exemplary sensor device is shown in the inset of FIG. 32,plate (b) for representation purposes. The I-V curves of theas-fabricated GaN NW two-terminal devices were non-linear andasymmetric. A small increase in the positive current after thedeposition of TiO₂ nanoclusters (curve 2) can be attributed to decreasedsurface depletion of the GaN NW due to passivation of surface states,and/or the high temperature deposition (325° C.) of the nanoclustersinitiating ohmic contact formation. The devices annealed at 700° C. for30 s after the deposition of TiO₂ NCs showed significant change in theirI-V characteristics with a majority of the devices exhibiting linear I-Vcurves. Interestingly, Pt NC deposition on TiO₂ coated GaN NWs furtherincreased the conductivity of the nanowire. This is due to the fact thatthe Pt clusters depleted the TiO₂ clusters by removing free electrons.Increased depletion in the TiO₂ clusters due to Pt would decrease TiO₂induced depletion in the GaN NW, leading to an increase in the NWcurrent. With the Pt/GaN hybrids, the current decreases followed by thedeposition of Pt (see FIG. 32, plate (b)) as expected due to thedepletion region formed in the NW under the metal clusters.

The nature of the depletion region formed by the nano-sized metalclusters on a semiconductor may be determined by Zhdanov's model. FIG.33 shows the calculated zero-bias depletion depth produced in GaN andTiO₂ respectively, as a function of the Pt cluster radius according toEquation (1). For calculating the depletion depth we assumed theeffective conduction band density of states in TiO₂ as 3.0×10²¹ cm⁻³ andpoint-defect related donor concentration as 1.0×10¹⁸ cm⁻³ [43,44]. Theelectron concentration in the GaN NWs was measured to be 1×10¹⁷ cm⁻³ ina separate experiment.

FIG. 33 indicates that even a single Pt NC of 2 nm radius cansignificantly deplete a 10 nm (average size) TiO₂ cluster. The effect ofTiO₂ depletion on GaN NW is difficult to determine as it could beinfluenced by numerous factors including interface states and particlegeometry. Given the very high density of TiO₂ clusters on the NW surface(see FIG. 31, plate (b)), it is clear that the Pt particles mostlyreside on the surfaces of TiO₂ NCs. However, from FIG. 33 we can seethat when Pt NCs are directly on GaN, they deplete the carriers in aneven larger region in the GaN NW. This qualitatively explains therelatively larger change in current observed when Pt NCs were depositedon bare GaN NWs as compared to the change in current when Pt NC weredeposit on the TiO₂-coated NWs.

Comparative Sensing Behavior of GaN/(TiO₂—Pt), GaN/Pt and GaN/TiO₂ NWNCHybrid Sensors

The photocurrent through the bare GaN NW devices did not change whenexposed to different chemicals mixed in air, even for concentrations ashigh as 3%. In contrast, the TiO₂-coated hybrid devices responded evento the pulses of 20 sccm airflow in the presence of UV excitation. Theresponse of the TiO₂ NC-coated GaN nanowire hybrid sensors to differentconcentrations of benzene, toluene, ethylbenzene, chlorobenzene, andxylene in air is discussed above. The GaN/TiO₂ hybrids showed noresponse when exposed to other chemicals such as alcohols, ketones,amides, alkanes, nitro/halo-alkanes, and esters.

Remarkably, after the deposition of Pt nanoclusters on the GaN/TiO₂hybrids, the sensors were no longer sensitive to benzene and otheraromatic compounds, but responded only to hydrogen, methanol, andethanol. In addition, the GaN/(TiO₂—Pt) hybrids showed no response whenexposed to higher carbon-containing (C>2) alcohols such as n-propanol,iso-propanol, and n-butanol. FIG. 5 shows the change of photocurrent ofa GaN/(TiO₂—Pt) sensor in the presence of 20 sccm air flow of air mixedwith 1000 μmol/mol (ppm) of methanol, ethanol, and water, respectively,and 20 sccm of nitrogen flow mixed with 1000 μmol/mol (ppm) hydrogen.The change in the photocurrent of the sensor when 20 sccm of breathingair is flowing through the test chamber serves as a reference forcalculating the sensitivity of the sensors. The sensitivity is definedas (R_(gas)−R_(air))/R_(air), where R_(gas) and R_(air) are theresistances of the sensor in the presence of the analyte-air mixture andin the presence of air only, respectively (R_(air) is replaced withR_(nitrogen) for hydrogen sensing experiments).

The GaN/TiO₂ hybrids without Pt showed no response to hydrogen and thealcohols. Interestingly, when Pt NC-coated GaN NW hybrids (GaN/Pt) withthe same nominal thickness were tested, they showed very limitedsensitivity only to hydrogen and not to any alcohols. The comparativesummary of the sensing behavior of the three different hybrids arepresented in FIG. 34.

The response of the GaN/(TiO₂—Pt) NWNC sensor to differentconcentrations of methanol in air is shown in FIG. 35, plate (a). FIG.35, plate (b) shows the response to different concentrations of hydrogenin nitrogen for the same GaN/(TiO₂—Pt) NWNC sensor device. The sensorresponse is much higher for hydrogen compared to methanol and ethanol.The response time is also much shorter for hydrogen as compared tomethanol, and the sensor photocurrent saturates after initial 20 sexposure.

The response time was defined as the time taken by the sensor current toreach 90% of the response (I_(f)−I₀) when exposed to the analyte. TheI_(f) is the steady sensor current level in the presence of the analyte,and I₀ is the current level without the analyte, which in our case is inthe presence of 20 sccm of air flow. The recovery time is the timerequired for the sensor current to recover to 30% of the response(I_(f)−I₀) after the gas flow is turned off (see Garzella C et al.(2000) Sensors and Actuators B: Chemical 68:189-196). The response timefor hydrogen was ≈60 seconds, whereas the response time for ethanol andmethanol was ≈80 seconds. The sensor recovery time for hydrogen was ≈45seconds and the recovery times for ethanol, methanol was ≈60 seconds and≈80 seconds, respectively. For comparison, Wang et al. demonstrated aconventional ZnO NW-based hydrogen sensor with a response time of 10minutes for 4.2% sensitivity (Wang H T et al. (2005) “Hydrogen-selectivesensing at room temperature with ZnO nanorods,” Appl. Phys. Lett.86:243503).

The sensitivity plot of a GaN/(TiO₂—Pt) hybrid device for the variousanalytes tested is shown in FIG. 36, plate (a). Note that the lowestconcentration detected for methanol and hydrogen (1 ppm or μmol/mol) isnot the sensor's detection limit, but a system limitation. It can beseen that the sensor is more sensitive to methanol than ethanol forconcentrations ≥1000 μmol/mol (ppm), and the relative sensitivityswitches for concentrations of 500 μmol/mol (ppm) and below. Similarbehavior is observed with twenty unique devices, possibly due todifference in surface coverage of the different alcohols over theconcentration range. FIG. 36, plate (b) is a comparative plot showingthe sensitivity of GaN/(TiO₂—Pt) and GaN/Pt hybrid sensors to hydrogenin nitrogen. The GaN/Pt hybrid devices showed relatively low sensitivitywith detection limit of 50 μmol/mol (ppm), below which the devicesstopped responding. The gas exposure time was also increased to 200seconds for the GaN/Pt devices to obtain increased response compared to100 seconds for the GaN/(TiO₂—Pt) GaN devices. The sensitivity of theGaN/(TiO₂—Pt) sensors was greater for alcohols and hydrogen whencompared with the same concentrations of water in air, which thusenables their use in high-humidity conditions.

Table X and Table XI compare the performance of the sensor devices ofthe present invention with sensors disclosed in the most recentliterature in terms of operation temperature, carrier gas, lowerdetection limit, and response/recovery times. The comparison indicatesthat the sensors devices of the present invention exhibit an excellentresponse to very low concentrations of analytes (100 ppb for ethanol and1 ppm for hydrogen) at room temperature, with air as the carrier gas.The testing conditions closely resembled real-life conditions, whichunderlines the significance of the disclosed sensors. The response andrecovery times were also lower for the disclosed sensors compared to theother conventional sensors, as shown in Tables X and XI.

TABLE X Performance of GaN/(TiO₂—Pt) NWNC hybrid sensors to ethanol incomparison with conventional sensors Lower Response/ Detection TestingRecovery Time Limit Carrier Gas Temperature Sensor of 80 s/75 s 100 ppbwith air Room Present 1% temperature (RT) Invention sensitivity⁴CNT¹/SnO₂ core  1 s/10 s 10 ppm air 300° C. shell nanostructuresMWCNTs²/ 20 s/20 s 18,000 ppm    air RT NaClO₄/ polypyrrole Metal-CNT ~2min/ 500 ppb with N₂ in a RT hybrids (recovery sensitivity <1% vacuumtest time not chamber reported) V₂O₅ nanobelts 50 s/50 s  5 ppm air 150°C.-400° C. ZnO nanorods 3.95 min/ 10 ppm Synthetic air 125° C.-300° C.5.3 min ZnO nanowires 10 s/55 s  1 ppm air 220° C. ITO³ nanowires 2 s/2s 10 ppm air 400° C. SnO₂ nanowires 2 s/2 s 10 ppm air 300° C. ¹Carbonnanotubes ²Multiwall carbon nanotubes ³Indium tin oxide ⁴Sensitivityvalues for sensors with lowest limit similar to disclosed results werecompared.

TABLE XI Performance of GaN/(TiO₂—Pt) NWNC hybrid sensors to hydrogen incomparison with conventional sensors Response/ recovery Testing timesLower detection limit Temperature Sensor of 60 s/45 s 1 ppm withsensitivity RT Present of 4% Invention CNT films 5 min/30 s  10 ppm RTSWCNT/SnO₂ 2 s/2 s 300 ppm 250° C. Pd/CNTs 5 min/5 min 30 ppm withsensitivity RT of 3% Pd/Si NWs 1 hr/50 min  3 ppm RT Pt doped SnO₂ 2min/10 min 100 ppm 100° C. NWs

The present results indicate the unique ability to tailor theselectivity of NWNC chemical sensors. With infinite combinations ofmetal and metal-oxide composite clusters available, there is a hugepotential for sensor designs targeted for a multitude of applications.

Example 3

Alcohol sensors using gallium nitride (GaN) nanowires (NWs)functionalized with zinc oxide (ZnO) nanoparticles are demonstrated.These sensors operate at room temperature, are fully recoverable, anddemonstrate a response and recovery time on the order of 100 seconds.The sensing is assisted by UV light within the 215-400-nm band and withthe intensity of 375 nW/cm² measured at 365 nm. The ability tofunctionalize an inactive NW surface, with analyte-specific activemetal-oxide nanoparticles, makes this sensor suitable for fabricatingmultianalyte sensor arrays.

Methods and Materials

Si-doped c-axis n-type GaN NWs were grown using catalyst-free molecularbeam epitaxy on Si (III) substrate as described in Bertness K A et al.(2008), supra, J. Cryst. Growth 310(13):3154-3158. The NW diameter andlength were in the ranges of 250-350 nm and 21-23 μm, respectively. TheGaN NWs were detached from the substrate by sonication in isopropanoland dielectrophoretically aligned across the pre-patterned electrodes.The electrodes were fabricated using photolithography followed bydeposition of a metal stack of Ti (40 nm)/Al (420 nm)/Ti (40 nm). Thickbottom electrodes ensure the free suspension of the NWs. For theformation of ohmic contacts to the NW ends, the top metal contacts werefabricated using a metal stack of Ti (70 nm)/Al (70 nm)/Ti (40 nm)/Au(40 nm), as described in A. Motayed et al. (2003), supra, J. Appl. Phys.93(2):1087-1094. Rapid thermal anneal (RTA) was performed at 700° C. for30 seconds in argon atmosphere to promote the formation of ohmiccontacts and to reduce the stress in the thick bottom electrodes.Finally, ZnO nanoparticles were sputter deposited on the NW device withan RF power of 300 W in 60 standard cubic centimeters per minute (sccm)of oxygen and 40 sccm of argon gas flow at room temperature. Depositiontime of 160 seconds was found to be optimal for the formation ofuncoalesced oxide nanoparticles.

The microstructure of the devices was characterized using a scanningelectron microscope (SEM) and X-ray diffraction (XRD). Due to the smallsize of the nanoparticles, the XRD signal from ZnO was not detected.Thus, the analysis was performed on a 300-nm-thick ZnO film sputterdeposited on Si (111) substrate with the assumption that the ZnOcrystallinity is similar for nanoparticles and for thin films depositedat the identical conditions. Current-voltage characteristics of thedevices were also measured to determine the nature of the NW-metalcontacts.

For the gas sensing measurements, a device was placed inside thestainless steel chamber with an inlet and an outlet for the analytevapors. The chamber, with a volume of 0.73 cm³, has a quartz window onthe top to facilitate exposure of the device to UV light. The wavelengthof the light bulb was confined to the range of 215-400 nm; the intensityrecorded at 365 nm was 3.75 nW/cm². The sensor baseline was establishedat a constant flow of 40 sccm of breathing air under illumination. Forsensing experiments, 40 sccm of the mixture of the breathing air andanalyte vapor was passed through the chamber. All sensing measurementswere performed in the presence of UV light and 5-V dc voltage biasapplied across the device terminals. Negligible or no chemiresistiveresponse was observed for all the chemicals in the absence of theillumination.

Results and Properties

FIG. 6, plate (a) shows a SEM image of a device with a single GaN NWsuspended across the metal electrodes. FIG. 6, plate (b) shows the ZnOnanoparticles on the facets of a GaN NW. The current-voltagecharacteristics of the device measured before and after RTA are shown inFIG. 6, plate (c). As shown in FIG. 6, plate (d), XRD reveals that thesputter-deposited ZnO is crystalline and highly (0002) textured.

Referring to FIG. 8 sensor response to air and nitrogen was evaluated.FIG. 8, plate (a) shows the device response to the different flow ratesof breathing air. As seen therein, device conductance decreases uponexposure to the breathing air, and the decrease is proportional to theflow rate. Opposite behavior (i.e., an increase in conductivity) isobserved when the device is exposed to nitrogen gas as seen in FIG. 8,plate (b).

Referring to FIG. 7, sensor response to alcohols and other analytes wasevaluated. When exposed to alcohol vapors (methanol, ethanol,n-propanol, isopropanol, n-butanol, and isobutanol), the devices showedan increase in conductivity with maximum sensitivity toward methanol.FIG. 7 shows the device response to 500-μmol/mol (ppm) methanol vapor inbreathing air.

For the isomers of an alcohol, the sensitivity decreases with branchingin the carbon chain. Hence, as shown in FIG. 7 (inset, bottom left), thesensitivity toward isobutanol is less than that toward n-butanol. Asshown in FIG. 7 (inset, bottom right), the devices show a negligibleresponse to possible interfering chemicals such as benzene and hexane,whereas the sensitivity toward 100 μmol/mol (ppm) of ethanol is similarto the sensitivity toward 1000 μmol/mol (ppm) of acetone. Ethanol vaporconcentration down to 100 nmol/mol (ppb) was successfully detected, andthe detection of even lower concentrations is possible with alternativemeasurement setup.

Example 4

A hybrid chemiresistive architecture, utilizing nanoengineeredwide-bandgap semiconductor backbone functionalized with multicomponentphotocatalytic nanoclusters of metal-oxides and metals was demonstrated.These sensors operated at room-temperature via photoenabled sensing.

Etching of Semiconductor Nanostructures

For real-time nanosensors, successful etching of semiconductingnanostructures, which is characterized by smooth surfaces with minimalsub-surface damage and appropriate side-wall profiles, is desired. Thisrequires overcoming the strong chemical bond energy in widegapsemiconductors, and also adjusting the process conditions to overcomeinherent defects in epitaxially grown films on non-native substratesusing heteroepitaxy. Otherwise, an un-optimized etching process mayresult in surface morphologies that include pits and/or pillars.

An Inductively Coupled Plasma-Reactive Ion Etching (ICP-RIE) processwith Cl₂/Ar/N₂ chemistry is provided, with an etch rate of about 100nm/min for GaN. The dry etching process may be optimized using X-rayphotoelectron spectroscopy (XPS), scanning electron microscopy (SEM),photoconductivity measurements, and photoluminescence (PL) measurements.

Fabrication Detail

Prior to dry etching, semiconductor wafer surfaces are treated withstandard RCA cleaning procedures. As a mask for selective etching, a500-nm-thick SiO₂ film is deposited by standard plasma-enhanced chemicalvapor deposition (PECVD). Etching patterns are defined by deep UVlithography using a proximity aligner capable of generating 300 nmfeature sizes. Electron beam deposition of Ni (˜20 nm) followed bylift-off is carried out to complete the formation of mask for the SiO₂etch.

Direct metal-masking of the semiconductor is not done in order to avoidun-intentional doping of the metal during the etch process. The ICP-RIEetching is performed using the following procedure. GaN etch isaccomplished using ICP etching with a Cl₂/N₂/Ar (25:5:2) gas mixtureunder a pressure of 5 mTorr with varying ICP etching power and radiofrequency (RF) power. For nitrides, Chlorine-based etches are usedbecause it has been shown to produce vertical sidewalls due to the ionassisted etching mechanism with smooth profiles. Temperature of the etchis a parameter that provides control of the sidewall angle. Withlow-temperature etch, the sub-surface damage may also be controlled.

Each sample is treated with a standard RCA clean before the activationannealing, the etching, and the measurements. Etching profile andsurface morphology may be investigated by SEM. The surface chemicalproperties of semiconductor after the etch is characterized using an XPSsystem and PL measurements performed at room temperature. The electricalproperties of etched semiconductor backbone are characterizedphotocurrent measurements. Photocurrent intensity is a direct measure ofthe surface recombination, i.e., higher photocurrent intensity willindicate less surface defect non-radiative recombination, hence lesssub-surface damage. For GaN, Ti/Al/Ti/Au (70 nm/70 nm/50 nm/50 nm) ohmicelectrodes are formed at both ends of the backbone nanostructures andthen annealed at temperatures from 500 C to 800 C for ˜1 min. Thenanodevices are then functionalized with different metal and metal-oxidenanoclusters using reactive sputtering.

A schematic representation of an exemplary fabrication flow forsemiconductor-nanocluster based gas sensors according to the presentinvention is shown in FIG. 37. As shown, the fabrication flow providesfor parallel architecture, with multiple parallel sections. Themulti-analyte arrays can be created on one single chip (10 mm×10 mm) bydepositing clusters of different components on different micro-scaledevices. This is possible due to low-temperature sputtering process usedfor the cluster deposition. An array of multiple sensors (e.g. fordetecting NO_(x), SO_(x), CO_(x), NH₃, and H₂O) may be fabricated all onone single chip. FIG. 38 shows exemplary inter-digitated GaN devices onSi and sapphire substrates formed using top-down processes (e.g., suchas shown in FIG. 37).

Example 5

Protection against explosive-based terrorism may be achieved bylarge-scale production of nano-sensor arrays that are inexpensive,highly sensitive and selective with low response and recovery times. Inthis study, the selective response of GaN nanowire/TiO₂ nanoclusterhybrids to nitroaromatic explosives, including trinitrotoluene (TNT),dinitrotoluene (DNT), nitrotoluene (NT), dinitrobenzene (DNB) andnitrobenzene (NB) at room temperature is demonstrated. The sensorsdetected between 0.5 ppb and 8 ppm TNT with good selectivity againstinterfering compounds such as toluene. The sensitivity of 1 ppm of TNTis ≅10% with response and recovery times of ≅30 seconds.

N-type (Si doped) GaN nanowires functionalized with TiO₂ nanoclusterswere utilized for selectively sensing nitro-aromatic explosivecompounds. GaN is a wide-bandgap semiconductor (3.4 eV) with uniqueproperties. Its chemical inertness and capability of operating inextreme environments (high-temperatures, presence of radiation, extremepH levels) is highly desirable for sensor design. TiO₂ is aphotocatalytic semiconductor with bandgap energy of 3.2 eV (anatasephase). The TiO₂ nanoclusters were selected to act as nanocatalysts toincrease the sensitivity, lower the detection time, and enable theselectivity of the structures to be tailored to a target analyte (e.g.,the most common explosives, trinitrotoluene (TNT) and othernitro-aromatics).

Materials and Methods

GaN nanowires were grown by Molecular Beam Epitaxy method as describedin Bertness K A et al. (2008), supra, J. Crystal Growth310(13):3154-3158. The nanowires are aligned on a pre-patternedsubstrate using dielectrophoresis. Details of the device fabrication arereported in Aluri G S et al. (2011) “Highly selective GaN-nanowire/TiO₂-nanocluster hybrid sensors for detection of benzene and relatedenvironment pollutants,” Nanotechnology 22(29):295503. After fabricationof two-terminal GaN NW devices, the TiO₂ NCs were deposited on the GaNNW surface using RF magnetron sputtering. The deposition was done at325° C. with 50 standard cubic centimeters per minute (sccm) of Ar flow,and 300 W RF power. The nominal deposition rate was about 0.24 Å/s.Thermal annealing of the complete sensor devices (GaN NW with TiO₂nanoclusters) was done at 700° C. for 30 seconds in a rapid thermalprocessing system. The device substrates, i.e., the sensor chips, werewire-bonded on a 24 pin ceramic package for the gas sensingmeasurements.

The microstructure and morphology of the sputtered TiO₂ films used forthe fabrication of the sensors were characterized by high-resolutiontransmission and scanning transmission electron microscopy (HRTEM/STEM),selected-area electron diffraction (SAED), and field-emission scanningelectron microscopy (FESEM). For the TEM characterization, the GaN NWswere dispersed on 10 nm thick carbon films supported by Mo-mesh grids,followed by the deposition of TiO₂ NCs and annealing and subsequent Ptdeposition. The samples were analyzed in a FEI Titan 80-300 TEM/STEMmicroscope operating at 300 kV accelerating voltage and equipped withS-TWIN objective lenses, which provided 0.13 nm (STEM) and 0.19 nm (TEM)resolution by points. The instrument also had a Gatan CCD imageacquisition camera, bright-field (BF), ADF and high-angle annulardark-field (HAADF) STEM detectors to perform spot, line profile, andareal compositional analyses using an EDAX 300 kV high-performance Si/LiX-ray energy dispersive spectrometer (XEDS).

The as-fabricated sensors were placed in a custom designed gas chamberfor gas exposure measurements. Detailed description of the experimentalsetup and experimental conditions is provided in Aluri G S et al.(2011), supra, Nanotechnology 22(29):295503. The device characterizationand the time dependent sensing measurements were done using an AgilentB1500A semiconductor parameter analyzer. The gas sensing experimentswere performed by measuring the electrical conductance of the devicesupon exposure to controlled flow of air/chemical mixture in presence ofUV excitation (25 W deuterium bulb operating in the 215 nm to 400 nmrange). For all the sensing experiments with chemicals, breathing air(<9 μmol/mol of water) was used as the carrier gas. After the sensordevices were exposed to the aromatic compounds, they were allowed toregain their baseline current with the air-chemical mixture turned-off,without purging or evacuating the test-chamber.

Results

Morphological and Structural Characterization of NWNC Hybrids

TEM imaging was conducted under minimal beam intensity conditions closeto the Scherzer defocus at highest available accelerating voltage of 300kV using both stationary beam (bright-field TEM/SAED, phase-contrasthigh-resolution TEM) and scanning beam (STEM/XEDS) modes. Areas foranalyses were selected near the wire's edges and on the amorphous carbonsupport film in the vicinity of the NWs. FIG. 39 shows HRTEM micrographsof a GaN NW on a thin amorphous carbon support films with TiO₂ coating.The deposited TiO₂ layer formed an island-like film, where 10 nm to 50nm partially aggregated particles (circled areas in FIG. 39) were ofteninterconnected into extended two-dimensional networks. This wasconsistent with SAED and compositional analyses of deposited TiO₂ filmsindicating a mixture of polycrystalline anatase and rutile phases.Despite the limited contrast difference between TiO₂ and GaN, detailedHRTEM and HR-STEM observations revealed 0.35 nm (101) hcp latticefringes belonging to anatase.

Current-Voltage (I-V) Characteristics of NWNC Hybrids

Referring to FIG. 40, I-V characteristics of a GaN NW two-terminaldevice at different stages of processing are shown. The I-V curves ofthe as-deposited devices were non-linear and asymmetric (with a lowcurrent of 35 nA). However, the current increased (to a 100 nA) with thedeposition of TiO₂ nanoclusters. This may be attributed to decreasedsurface depletion of the GaN NW due to passivation of surface states,and/or the high temperature deposition (325° C.) of the nanoclustersinitiating ohmic contact formation. The devices annealed at 700° C. for30 seconds showed significant changes in their I-V characteristics witha majority of the devices exhibiting linear I-V curves. This isconsistent given low resistance ohmic contacts to the nitrides requireannealing at 700° C.-800° C.

Sensing Behavior of GaN/TiO₂ NWNC Hybrid Sensors

The photocurrent through the bare GaN NW devices did not change whenexposed to different chemicals mixed in air, even for concentrations ashigh as 3%. In contrast, the TiO₂-coated hybrid devices responded evento the pulses of 20 sccm airflow in the presence of UV excitation. Theresponse of the TiO₂ NC-coated GaN nanowire hybrid sensors to differentconcentrations of benzene, toluene, ethylbenzene, chlorobenzene, andxylene in air is discussed above. The GaN/TiO₂ hybrids showed noresponse when exposed to other chemicals such as alcohols, ketones,amides, alkanes, nitro/halo-alkanes, and esters.

The response of the TiO₂ coated hybrid devices when exposed to aconcentration of 100 ppb of the aromatics and nitro-aromatics in air canis shown in FIG. 41, plate (a). The photocurrent for these sensorsincreased with respect to air when exposed to toluene vapors, whereasfor every other aromatic compound the photocurrent decreased relative toair. The response is observed to increase with the increase in thenumber of nitro groups attached to the aromatic compound. The responseof the hybrid device to different concentrations of TNT in air from 8ppm down to as low as 500 ppt is shown in FIG. 41, plate (b). Theresponse time is defined as the time taken by the sensor current toreach 90% of the response (I_(f)−I₀) when exposed to the analyte. TheI_(f) is the steady sensor current level in the presence of the analyte,and t₀ is the current level without the analyte, which in this case isin the presence of air. The recovery time is the time required for thesensor current to recover to 30% of the response (I_(f)−I₀) after thegas flow is turned off. The response and recovery times of thenano-devices to different concentrations of TNT are seconds. Theresponse and recovery times of the rest of the compounds varied fromseconds to seconds.

The sensitivity is defined as (R_(gas)−R_(air))/R_(air), where R_(gas)and R_(air) are the resistances of the sensor in the presence of thechemical-air mixture and in presence of air, respectively. Thesensitivity plot of a hybrid device for the different aromatics andnitro-aromatics tested is shown in FIG. 42. The sensitivity((R_(gas)−R_(air))/R_(air)) for 1 ppm of TNT is ≅10%. The devicesexhibit a very highly sensitive and selective response to TNT whencompared to interfering compounds like toluene. Toluene shows anincrease in response with respect to air, whereas TNT shows a decreasewhen compared to air. The plot identifies the sensor's ability to sensewide concentration ranges of the indicated chemicals. The sensitivity oftwo different devices (device 1—D1; device 2—D2) to the differentaromatic compounds can be seen in FIG. 43.

As discussed above, oxygen vacancy defects (Ti³⁺ sites) on the surfaceof TiO₂ are the “active sites” for the adsorption of species likeoxygen, water, and organic molecules. In the presence of UV excitationwith an energy above the bandgap energy of anatase TiO₂ (3.2 eV) and GaN(3.4 eV), electron-hole pairs are generated both in the GaN NW and inthe TiO₂ cluster. Photogenerated holes in the nanowire tend to diffusetowards the surface due to surface band bending. This effect ofseparation of photogenerated charge carriers results in a longerlifetime of photogenerated electrons, which in turn enhances thephotoresponse of the nanowire devices in general. Since thenitro-aromatic compounds are highly electronegative, they tend toattract electrons from other molecules through charge transfer. Thischarge transfer between the adsorbed species on the TiO₂ nanocluster,and the nitro groups in the nitro-aromatic compounds increases the widthof the depletion region in the nanowire device, reducing the current.

The potential of the disclosed nanostructure-nanocluster hybrids fornext-generation nano-sensors having the capability to detect explosivecompounds quickly and reliably is clearly demonstrated. The GaN/TiO₂nanowire nanocluster hybrid devices tested detected trace amounts ofaromatic and nitro-aromatic compounds in air at room temperature withvery low response and recovery times (≅30 seconds). The nitro-aromaticexplosives like TNT are selectively detectable even for concentrationsas low as 500 ppt.

Example 6

Nitrogen dioxide (NO₂) sensors using gallium nitride (GaN) nanowires(NWs) functionalized with titanium dioxide (TiO₂) nanoclusters aredemonstrated. Exemplary sensor fabrication methodologies are describedabove (e.g., see Example 1 & FIG. 19).

FIG. 44, plate (a) illustrates the dynamic responses of the TiO₂ basedsensor exposed to 250 ppm NO₂ mixed with breathing air under UVillumination and dark, and at room temperature. For each cycle, the gasexposure time was 300 s. FIG. 44, plate (b) illustrates change inresistance under UV at mixtures of 100 ppm, 250 ppm, and 500 ppm withbreathing air, with the inset showing the measured responses under UV asa function of NO₂ concentrations with uncertainty. Sensitivity S ispresented by (I_(g)−I_(α))×100/I_(α), wherein I_(g) is the devicecurrent in the presence of an analyte in breathing air and Iα is thecurrent in pure breathing air, both measured 300 s after the flow isturned on. FIG. 45 illustrates schematically an NO₂ gas sensingmechanism of the TiO₂ sensor under UV illumination and at roomtemperature. FIG. 45, plate (a) shows the mechanism in a darkenvironment with breathing air in. FIG. 45, plate (b) shows themechanism under UV illumination in breathing air. FIG. 45, plate (c)shows the mechanism under UV illumination with a mixture of NO₂ andbreathing air.

The response of the TiO₂ based sensor exposed to 500 ppm NO₂ under UVillumination and under dark at room temperature is shown in FIG. 46. Asdescribed, the UV illumination allows for efficient photodesorption ofadsorbed oxygen and hydroxyl species, thus introducing additional oxidesurface sites or receptors for adsorption of target molecules.Photocatalytic reactions then occur between the adsorbed targetmolecules and the photo carriers in the oxide sites, leading to amodification of the surface potential and semiconductor backbone currentchange (transduction). The photodesorption of the adsorbed targetmolecules and reaction species leads to a reversal of photocurrent tobaseline (recovery).

A GIXRD scan of thermally processed ultrathin TiO₂ film is shown in FIG.47, plate (a). Optical properties (bandgap) are illustrated in FIG. 47,plate (b).

Example 7

Carbon dioxide (CO₂) sensors using gallium nitride (GaN) nanowires (NWs)functionalized with tin oxide and copper oxide (SnO₂—CuO) nanoclustersare demonstrated. Exemplary sensor fabrication methodologies aredescribed above.

FIG. 48, plate (a) illustrates schematically a SnO₂—Cu nanocluster CO₂sensor, including an electrode disposed on a sapphire substrate, and GaNNWs functionalized with SnO₂ nanoclusters and SnO₂—CuO nanoclusters. AFMimages of the SnO₂—Cu nanocluster CO₂ sensor are shown in FIG. 48,plates (b) and (c). FIG. 49 illustrates the dynamic response of theSnO₂—Cu based sensor exposed to CO₂ at room temperature for variousconcentrations. FIG. 50 illustrates graphically the response of the SnO₂based sensor at different relative humidity (RH) concentrations at roomtemperature.

All publications and patents mentioned in this specification are hereinincorporated by reference to the same extent as if each individualpublication or patent application was specifically and individuallyindicated to be incorporated by reference in its entirety. While theinvention has been described in connection with specific embodimentsthereof, it will be understood that it is capable of furthermodifications and this application is intended to cover any variations,uses, or adaptations of the invention following, in general, theprinciples of the invention and including such departures from thepresent disclosure as come within known or customary practice within theart to which the invention pertains and as may be applied to theessential features hereinbefore set forth.

What is claimed is:
 1. A method for manufacturing one or morenanoparticle gas sensors on a single chip, which sensors are configuredto detect at least one type of gas, the method comprising: providing afirst semiconductor layer on top of a substrate layer; transferring anetching pattern to the first semiconductor layer; etching the firstsemiconductor layer to form a plurality of semiconductor electrodes; anddepositing either (a) first metal oxide nanoparticles and second metaloxide nanoparticles on a first subset of the plurality of semiconductorelectrodes or (b) third metal oxide nanoparticles and first metalnanoparticles on the first subset of semiconductor electrodes, togenerate a layer for at least some of the one or more nanoparticle gassensors which adsorbs a first type of gas and a first interferingcompound.
 2. The method of claim 1, wherein the one or more nanoparticlegas sensors on the single chip are a same type of gas sensor configuredto detect a single type of gas.
 3. The method of claim 1, wherein thefirst semiconductor layer is made from GaN.
 4. The method of claim 1,wherein the one or more nanoparticle sensors on the single chip detectone or more of NO_(x), SO_(x), CO_(x), NH₃ and H₂O, where x is aninteger value.
 5. The method of claim 1, wherein the step of etching thefirst semiconductor layer is performed using at least one of reactiveion etching (RIE) or wet chemical etching.
 6. The method of claim 1,wherein a buffer layer is provided between the first semiconductor layerand the substrate layer.
 7. The method of claim 1, wherein all of thenanoparticles have a diameter of less than 200 nm.
 8. The method ofclaim 1, wherein the substrate layer is made from one of Si andSapphire.
 9. The method of claim 1, wherein the at least one type of gasis at least two types of gases, the method further comprising:depositing either (c) fourth metal oxide nanoparticles and fifth metaloxide nanoparticles on a second subset of the plurality of semiconductorelectrodes or (d) sixth metal oxide nanoparticles and second metalnanoparticles on the second subset of semiconductor electrodes, togenerate a layer for the one or more nanoparticle gas sensors whichadsorbs a second type of gas and a second interfering compound.
 10. Themethod of claim 9, wherein the one or more nanoparticle sensors on thesingle chip detect two of NO_(x), SO_(x), CO_(x), NH₃ and H₂O where x isan integer value.
 11. The method of claim 1, wherein said one or morenanoparticle sensors exhibit a change in output upon detection of saidat least one type of gas, said output selected from the group consistingof current, voltage and resistance.
 12. The method of claim 1, whereinsaid one or more nanoparticle sensors enable detection of said at leastone type of gas within a carrier gas of air, nitrogen or argon.
 13. Themethod of claim 1, wherein said one or more nanoparticle sensors exhibitincreased conductivity upon exposure to said at least one type of gas inthe presence of UV excitation.
 14. A method for manufacturing one ormore gas sensors on a single chip, which sensors are configured todetect at least one type of gas, the method comprising: providing afirst semiconductor layer on top of a substrate layer; transferring anetching pattern to the first semiconductor layer; etching the firstsemiconductor layer to form a plurality of semiconductor electrodes; anddepositing either (a) first metal oxide particles and second metal oxideparticles on a first subset of the plurality of semiconductor electrodesor (b) third metal oxide particles and first metal particles on thefirst subset of semiconductor electrodes, to generate a layer for atleast some of the one or more particle gas sensors which adsorbs a firsttype of gas and a first interfering compound wherein all of theparticles have a diameter of less than 200 nm.
 15. The method of claim14, wherein the one or more gas sensors on the single chip are a sametype of gas sensor configured to detect a single type of gas.
 16. Themethod of claim 14, wherein the first semiconductor layer is made fromGaN.
 17. The method of claim 14, wherein the one or more sensors on thesingle chip detect one or more of NO_(x), SO_(x), CO_(x), NH₃ and H₂Owhere x is an integer value.
 18. The method of claim 14, wherein thestep of etching the first semiconductor layer is performed using atleast one of reactive ion etching (RIE) or wet chemical etching.
 19. Themethod of claim 14, wherein a buffer layer is provided between the firstsemiconductor layer and the substrate layer.
 20. The method of claim 14,wherein the substrate layer is made from one of Si and Sapphire.
 21. Themethod of claim 14, wherein the at least one type of gas is at least twotypes of gases, the method further comprising: depositing either (c)fourth metal oxide particles and fifth metal oxide particles on a secondsubset of the plurality of semiconductor electrodes or (d) sixth metaloxide particles and second metal particles on the second subset ofsemiconductor electrodes, to generate a layer for the one or moreparticle gas sensors which adsorbs a second type of gas and a secondinterfering compound.
 22. The method of claim 21, wherein the one ormore particle sensors on the single chip detect two of NO_(x), SO_(x),CO_(x), NH₃ and H₂O, where x is an integer value.
 23. The method ofclaim 14, wherein said one or more particle sensors exhibit a change inoutput upon detection of said at least one type of gas, said outputselected from the group consisting of current, voltage and resistance.24. The method of claim 14, wherein said one or more particle sensorsenable detection of said at least one type of gas within a carrier gasof air, nitrogen or argon.
 25. The method of claim 14, wherein said oneor more particle sensors exhibit increased conductivity upon exposure tosaid at least one type of gas in the presence of UV excitation.